Use VTTScanner for VTT cue settings parsing

Transcribe VTTCue::parseSettings (and helpers) into using VTTScanner
to perform the parsing of cue settings.

BUG=305317

Review URL: https://codereview.chromium.org/134153002

git-svn-id: svn://svn.chromium.org/blink/trunk@164955 bbb929c8-8fbe-4397-9dbb-9b2b20218538