Generate and assign media track ids in demuxers.
Currently media track ids are generated on blink level, but this
complicates media track management. In particular when media tracks are
created on demuxer level we don't know their unique ids yet, so we
can't easily set up track id -> demuxer stream mappings. Assigning
track ids at the demuxer level makes that much easier.
BUG=249427,341581
Review-Url: https://codereview.chromium.org/2050043002
Cr-Commit-Position: refs/heads/master@{#400198}
28 files changed
