Rename TextTrackRegion/TextTrackRegionList to VTTRegion/VTTRegionList

http://dev.w3.org/html5/webvtt/#vttregion-interface
http://dev.w3.org/html5/webvtt/#vttregionlist-interface

This is a completely mechanical rename, except for added instanceof
tests and removing NoInterfaceObject (not per spec) from VTTRegionList
to make them pass.

Silvia (the spec editor) says in the bug that the spec hasn't changed
since the implementation, so together with the fact that this is
RuntimeEnabled, it should be safe despite being Web facing.

BUG=313604

Review URL: https://codereview.chromium.org/55653003

git-svn-id: svn://svn.chromium.org/blink/trunk@161209 bbb929c8-8fbe-4397-9dbb-9b2b20218538