Move code that applies to audio and video tracks into TrackBase.

I also moved the EventTarget code into TextTrack, because audio and video
tracks aren't event targets.

BUG=249427

Review URL: https://codereview.chromium.org/92783002

git-svn-id: svn://svn.chromium.org/blink/trunk@165093 bbb929c8-8fbe-4397-9dbb-9b2b20218538