Re: [mediacapture-record] Clarification needed for mime type handling (#170)

I also made [a slightly different fiddle](https://jsfiddle.net/pehrsons/bmfuo2yr/). This one creates the `MediaRecorder` with an empty `MediaStream`, but adds the tracks before `start()`, to show some of the fundamental problems of setting the mime type and the tracks at different times.

Firefox:
```
[MediaRecorder "" [audio]]: ctor-mime: ""; recording-mime: "audio/ogg"; blob-mime: "audio/ogg"; recorded tracks: [audio]
[MediaRecorder "" [video]]: ctor-mime: ""; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [video]
[MediaRecorder "" [audio,video]]: ctor-mime: ""; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [audio,video]
[MediaRecorder "audio/webm;codecs=opus" [audio]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "audio/ogg"; blob-mime: "audio/ogg"; recorded tracks: [audio]
[MediaRecorder "audio/webm;codecs=opus" [video]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [video]
[MediaRecorder "audio/webm;codecs=opus" [audio,video]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [audio,video]
[MediaRecorder "video/webm;codecs=vp8" [audio]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "audio/ogg"; blob-mime: "audio/ogg"; recorded tracks: [audio]
[MediaRecorder "video/webm;codecs=vp8" [video]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [video]
[MediaRecorder "video/webm;codecs=vp8" [audio,video]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [audio,video]
[MediaRecorder "video/webm;codecs=opus,vp8" [audio]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "audio/ogg"; blob-mime: "audio/ogg"; recorded tracks: [audio]
[MediaRecorder "video/webm;codecs=opus,vp8" [video]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [video]
[MediaRecorder "video/webm;codecs=opus,vp8" [audio,video]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "video/webm"; blob-mime: "video/webm"; recorded tracks: [audio,video]
[MediaRecorder "audio/webm;codecs=opus,vp8" [audio]]: Exception; NotSupportedError: Operation is not supported
[MediaRecorder "audio/webm;codecs=opus,vp8" [video]]: Exception; NotSupportedError: Operation is not supported
[MediaRecorder "audio/webm;codecs=opus,vp8" [audio,video]]: Exception; NotSupportedError: Operation is not supported
```

Chrome:
```
[MediaRecorder "" [audio]]: ctor-mime: ""; recording-mime: ""; blob-mime: ""; recorded tracks: [audio]
[MediaRecorder "" [video]]: ctor-mime: ""; recording-mime: ""; blob-mime: ""; recorded tracks: [video]
[MediaRecorder "" [audio,video]]: ctor-mime: ""; recording-mime: ""; blob-mime: ""; recorded tracks: [audio,video]
[MediaRecorder "audio/webm;codecs=opus" [audio]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "audio/webm;codecs=opus"; blob-mime: "audio/webm;codecs=opus"; recorded tracks: [audio]
[MediaRecorder "audio/webm;codecs=opus" [video]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "audio/webm;codecs=opus"; blob-mime: "audio/webm;codecs=opus"; recorded tracks: [video]
[MediaRecorder "audio/webm;codecs=opus" [audio,video]]: ctor-mime: "audio/webm;codecs=opus"; recording-mime: "audio/webm;codecs=opus"; blob-mime: "audio/webm;codecs=opus"; recorded tracks: [audio,video]
[MediaRecorder "video/webm;codecs=vp8" [audio]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "video/webm;codecs=vp8"; blob-mime: "video/webm;codecs=vp8"; recorded tracks: [audio]
[MediaRecorder "video/webm;codecs=vp8" [video]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "video/webm;codecs=vp8"; blob-mime: "video/webm;codecs=vp8"; recorded tracks: [video]
[MediaRecorder "video/webm;codecs=vp8" [audio,video]]: ctor-mime: "video/webm;codecs=vp8"; recording-mime: "video/webm;codecs=vp8"; blob-mime: "video/webm;codecs=vp8"; recorded tracks: [audio,video]
[MediaRecorder "video/webm;codecs=opus,vp8" [audio]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "video/webm;codecs=opus,vp8"; blob-mime: "video/webm;codecs=opus,vp8"; recorded tracks: [audio]
[MediaRecorder "video/webm;codecs=opus,vp8" [video]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "video/webm;codecs=opus,vp8"; blob-mime: "video/webm;codecs=opus,vp8"; recorded tracks: [video]
[MediaRecorder "video/webm;codecs=opus,vp8" [audio,video]]: ctor-mime: "video/webm;codecs=opus,vp8"; recording-mime: "video/webm;codecs=opus,vp8"; blob-mime: "video/webm;codecs=opus,vp8"; recorded tracks: [audio,video]
[MediaRecorder "audio/webm;codecs=opus,vp8" [audio]]: Exception; NotSupportedError: Failed to construct 'MediaRecorder': Failed to initialize native MediaRecorder the type provided (audio/webm;codecs=opus,vp8) is not supported.
[MediaRecorder "audio/webm;codecs=opus,vp8" [video]]: Exception; NotSupportedError: Failed to construct 'MediaRecorder': Failed to initialize native MediaRecorder the type provided (audio/webm;codecs=opus,vp8) is not supported.
[MediaRecorder "audio/webm;codecs=opus,vp8" [audio,video]]: Exception; NotSupportedError: Failed to construct 'MediaRecorder': Failed to initialize native MediaRecorder the type provided (audio/webm;codecs=opus,vp8) is not supported.
```

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Received on Wednesday, 19 June 2019 09:31:40 UTC