Decouple tokenizer/tree builder state from WebVTTParser

Decouple tokenizer/tree builder state from WebVTTParser

The WebVTTTokenizer owned by the WebVTTParser (m_tokenizer) is only
used by the WebVTTParser::createDocumentFragmentFromCueText method
on WebVTTParser.
The same is true for other state like the current token (m_token),
the current (subtree root) node (m_currentNode) and the stack of
language tags (m_languageStack).
Transplant this state together with it's mutator method -
constructTreeFromToken - onto a new helper class, WebVTTTreeBuilder,
and then use that in WebVTTParser::createDocumentFragmentFromCueText.

With the above in place, WebVTTParser::createDocumentFragmentFromCueText
can be made static, avoiding the contorted (and unnecessary) allocation
in TextTrackCue::createWebVTTNodeTree.

BUG=305317
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=161073

[Nate Chapin, 2013-10-31 16:42:07]

LGTM!

[commit-bot: I haz the power, 2013-10-31 17:00:11]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/fs@opera.com/52713005/1

[commit-bot: I haz the power, 2013-10-31 18:00:19]

Change committed as 161073