Add a use counter for VTTCue right-to-left rendering

The WebVTT position model has been changed:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20037

In order to ship this (or any alternative) we need to know that it
doesn't break too much, and right-to-left text used to flip the
interpretation of the position setting, but doesn't any longer.

BUG=none

Review URL: https://codereview.chromium.org/276193002

git-svn-id: svn://svn.chromium.org/blink/trunk@173841 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Tighten the asserts in VTTCue::calculateDisplayParameters()

BUG=none

Review URL: https://codereview.chromium.org/276183002

git-svn-id: svn://svn.chromium.org/blink/trunk@173842 bbb929c8-8fbe-4397-9dbb-9b2b20218538