Re: [mediacapture-fromelement] The sequence canvas.transferControlToOffscreen() followed by canvas.captureStream() does not appear to be specified (#84)

Chromium does increment `currentTime` and fire `timeupdate` when more than one bitmap is transferred to the `OffscreenCanvas` (https://plnkr.co/edit/ptYxrRZTbc7f3A4zVDXK?p=preview)

```
    const canvas = document.querySelector("canvas");
    const video = document.querySelector("video");
    video.ontimeupdate = e => console.log(video.currentTime);
    const offscreen = canvas.transferControlToOffscreen();
    const stream = canvas.captureStream();
    const [track] = stream.getTracks();
    track.onmute = track.onunmute = e => console.log(e);
    video.srcObject = stream;
```

in `Worker`

```
    setTimeout(async() => {
      const request = await (await fetch(dataURL)).blob();
      const bitmap = await createImageBitmap(request);
      osctx.transferFromImageBitmap(bitmap);
    }, 1000 / 30);
```

Should `currentTime` of HTML `<video>` be incremented when the first bitmap is transferred to the `OffscreenCanvas`, if only by 1/30 of 1 second?

Firing of `mute` event is helpful in this case.

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