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This change:
1) Implements WebSourceBufferImpl::AppendChunks(), plumbing the appended
   chunks through ChunkDemuxer into the stream parser's ProcessChunks().

2) Implements WebCodecsEncodedChunkStreamParser::ProcessChunks(),
   emitting cached config when necessary (prior to calling the
   NewBuffersCB, initially, or if a flush() operation had just happened
   before the chunks were appended.

3) Aligns as much as possible with existing architecture for similar
   handling by regular bytestream parsers, for improved reuse and
   maintainability.

4) Adds a basic end-to-end vp9 video chunk buffering and playback test.

Later refinements will add more tests and may improve usability with
things like EncodedAudioChunk duration, EncodedVideoChunk
decodeTimestamp, and h264 support (where applicable by build flags).

BUG=1144908

Change-Id: Ib787d22b0171826fa28bdbb776410e02e25a9cba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575501
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836204}