Make VTTRegion::setRegionSettings use VTTScanner

Next step in the transition of VTTRegion parsing code to VTTScanner.
Convert VTTRegion::setRegionSettings, and fold VTTRegion::parseSetting
into it.

BUG=305317

Review URL: https://codereview.chromium.org/140503004

git-svn-id: svn://svn.chromium.org/blink/trunk@165399 bbb929c8-8fbe-4397-9dbb-9b2b20218538