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This change:

1) Implements SourceBuffer::appendEncodedChunks(). This is the first
   promise-based MSE API. Like async appendBuffer(), once the
   synchronous prepareAppend steps are complete, an async task is
   scheduled to complete the chunks' append. Unlike async
   appendBuffer(), the 'update', 'updateend', 'abort', 'error' events
   involved in an async chunk append are not enqueued, as the same
   information is exposed instead in promise rejections. Also, mixing
   event-notification with promise resolution/rejection is confusing and
   redundant, and w3ctag guidelines for promise-based APIs need such
   events originating in a promise's async execution to be dispatched
   (not just enqueued) prior to promise resolution/rejection occurring.

2) Converts (using new local helpers in SourceBuffer) the chunks
   directly into the type used in the underlying MSE buffering
   implementation (StreamParserBuffers in a circular_deque) during the
   synchronous portion of appendEncodedChunks().

3) The async chunk task is canceled during contextDestruction; it also
   is used to gate hasPendingActivity. Essentially, it's a 3rd async
   operation the SourceBuffer could do (in addition to pre-existing
   async appendBuffer and async remove). At most one of these three
   async operations may be pending at a time. This keeps the behavior
   unsurprising and aligns better w.r.t. previous similar async MSE
   operations.

4) Adds a new WebSourceBuffer AppendChunks method and a stubbed
   implementation of it in WebSourceBufferImpl.

Later changes will update WSBI::AppendChunks() to send the buffers
through ChunkDemuxer to the WebCodecsEncodedChunkStreamParser, and add
tests for promise rejection/abort/success scenarios and basic
end-to-end buffering and playback of encoded chunks with MSE.
Refinements, such as supporting h264 chunk buffering, using
non-hardcoded audio chunk duration (from a new optional duration, but
required-for-MSE, EncodedAudioChunkInit attribute), and letting the app
provide decode timestamp in EncodedVideoChunkInit, may also come later.

BUG=1144908

Change-Id: Ieb5d0942e68f48156bee9290dcb99dad2e280e85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574534Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836201}