[webrtc-encoded-transform] Expose MIME type as frame metadata (#158)

alvestrand has just created a new issue for https://github.com/w3c/webrtc-encoded-transform:

== Expose MIME type as frame metadata ==
At the moment, payload type is exposed in metadata. But decoding this information needs a) access to SDP, and b) an SDP parser that can accurately identify the relevant MIME type.
The platform already knows the MIME type, so supplying it should be simple. This makes the API user's task easier, and seems to have little downside.


Please view or discuss this issue at https://github.com/w3c/webrtc-encoded-transform/issues/158 using your GitHub account


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