Implement AudioTrack, AudioTrackList, VideoTrack, and VideoTrackList

This is based on Aaron Colwell's patch:
https://codereview.chromium.org/170233009/#ps270001

BUG=249427
TEST=LayoutTests/media/avtracklists.html

Review URL: https://codereview.chromium.org/284513003

git-svn-id: svn://svn.chromium.org/blink/trunk@176224 bbb929c8-8fbe-4397-9dbb-9b2b20218538