Use VTTScanner for VTT region settings parsing

Start transitioning the VTTRegion settings parsing code to use
VTTScanner, by modifying VTTRegion::parseSetting (the "name=value pair"-
part of the parsing) to use VTTScanner.

Remove VTTParser::collectWord since it becomes unused.

BUG=305317

Review URL: https://codereview.chromium.org/137033002

git-svn-id: svn://svn.chromium.org/blink/trunk@165149 bbb929c8-8fbe-4397-9dbb-9b2b20218538