RTCEncodedAudioFrame and RTCEncodedVideoFrame constructors handling of changed metadata

[youennf]

Currently, the spec allows changing all metadata to any value.

Looking at what Chrome does, it validates a lot the metadata.
For instance, for audio frames, it seems that only RTP timestamp can be changed.

Either the constructor ignores other metadata or it throws.

I wonder whether:

1. We should not have dictionaries for modifiable audio and video metadata. For audio, only RTP timestamp would be listed in that dictionary.
2. We add validation rules as needed.

[guidou]

Note that the Chrome behavior is in many cases buggy. Some operations throw because they haven't been implemented yet, not because they shouldn't work.
But there might be some metadata fields where it makes sense to reject setting a value.