Introduce a RTCEncodedFrameMetadata dictionary to share more definitions between audio and video metadata

[youennf]

We could introduce:

dictionary RTCEncodedFrameMetadata {
    unsigned long synchronizationSource;
    octet payloadType;
    sequence<unsigned long> contributingSources;
    unsigned long rtpTimestamp;
    DOMHighResTimeStamp receiveTime;
    DOMHighResTimeStamp captureTime;
    DOMHighResTimeStamp senderCaptureTimeOffset;
    DOMString mimeType;
};
dictionary RTCEncodedAudioFrameMetadata : RTCEncodedFrameMetadata {
    ...
};
dictionary RTCEncodedVideoFrameMetadata : RTCEncodedFrameMetadata {
    ...
};

Similarly, for RTCEncodedAudioFrame and RTCEncodedVideoFrame, we could introduce:

interface mixing RTCEncodedFrame {
    attribute ArrayBuffer data;
};
RTCEncodedAudioFrame includes RTCEncodedFrame;
RTCEncodedVideoFrame includes RTCEncodedFrame;