Remove TextTrack.regions and VTTRegionList

Removed from the WebVTT spec https://github.com/w3c/webvtt/pull/31

Because we no longer need to track a list of regions in TextTrack, we
can also remove all the plumbing between the parser and the (loadable)
TextTrack.

BUG=690014

Review-Url: https://codereview.chromium.org/2685943004
Cr-Commit-Position: refs/heads/master@{#449791}
19 files changed