[mediacapture-record] Should MediaRecorderOptions fields be optional?

eliasmeire has just created a new issue for https://github.com/w3c/mediacapture-record:

== Should MediaRecorderOptions fields be optional? ==
`MediaRecorderOptions` is defined as:

```webidl
dictionary MediaRecorderOptions {
  DOMString mimeType;
  unsigned long audioBitsPerSecond;
  unsigned long videoBitsPerSecond;
  unsigned long bitsPerSecond;
};
```

Shouldn't all the members be optional/nullable?

```webidl
dictionary MediaRecorderOptions {
  DOMString? mimeType;
  unsigned long? audioBitsPerSecond;
  unsigned long? videoBitsPerSecond;
  unsigned long? bitsPerSecond;
};
```
or should all members be specified when giving options to the MediaRecorder constructor?

Please view or discuss this issue at https://github.com/w3c/mediacapture-record/issues/156 using your GitHub account