Allow muting/unmuting audio through media track API
This CL implements muting/unmuting audio via HTML5 media track APIs.
More specifically it adds basic plumbing to deliver media track state
changes from blink level to media::Renderer, by translating blink track
ids into DemuxerStream pointers in demuxer and then delivering
DemuxerStream state information to media renderers. For now audio
stream is disabled by setting volume to 0, but this can be done more
efficiently in the future.
BUG=249427
Review URL: https://codereview.chromium.org/1812543003
Cr-Commit-Position: refs/heads/master@{#387789}
22 files changed
