Basic media tracks implementation for media stream parsers

This CL adds basic media::MediaTrack implementation, media::MediaTracks
collection that associates media tracks with audio/video configs, and
makes necessary changes in MSE stream parsers to pass around the
MediaTracks collection instead of individual audio/video configs.

BUG=249427, 249428

Review URL: https://codereview.chromium.org/1716503002

Cr-Commit-Position: refs/heads/master@{#378583}
21 files changed