Improve caption button behavior for video player.
- If there is only one caption, the CC button should just
toggle it on and off.
- If the track has no label, instead of showing Unknown,
it should show the language label and if that is also
empty, it should show Track X.
BUG=
648696
Committed:
https://crrev.com/480c4553694769458606f287350d13ff144a05e6
Cr-Commit-Position: refs/heads/master@{#429044}