Pass a VTTScanner to VTTRegion::parseSettingValue

Complete the transition to using VTTScanner for VTTRegion, by pushing
a VTTScanner-reference down into VTTRegion::parseSettingValue, and make
the different "subparsers" use it instead of a locally declared one.

BUG=305317

Review URL: https://codereview.chromium.org/144893002

git-svn-id: svn://svn.chromium.org/blink/trunk@165526 bbb929c8-8fbe-4397-9dbb-9b2b20218538