Assert some invariants in TextTrack

A cue having a track reference implies it being in that track's list of
cues. Rely on this in a few places.

BUG=321654

Review URL: https://codereview.chromium.org/979743003

git-svn-id: svn://svn.chromium.org/blink/trunk@191292 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed
tree: cf667283dc8c1f16fcd0da14b1bf81500e87418c
  1. third_party/