Use fancy new Vector-based String constructors in the WebVTT parser
https://bugs.webkit.org/show_bug.cgi?id=109619

Reviewed by Benjamin Poulain.

No change in behavior. Added some FIXMEs for future perf optimization.

* html/track/WebVTTParser.cpp:
(WebCore::WebVTTParser::constructTreeFromToken):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142772 268f45cc-cd09-0410-ab3c-d52691b4dbfc