Turn VTTToken into an immutable object

Turn VTTToken into an immutable object

With the data-handling in VTTTokenizer::nextToken simplified to use
fewer entrypoints on VTTToken, it becomes easier to simply remove
intermediate buffers/StringBuilders from the token type, and instead
store them on the stack of nextToken().

Remove appendToData() from VTTToken, and replace it with append()s
on a local StringBuilder instead. Treat addNew{Class,Annotation}
similarly. The "new" StringBuilders are named as the corresponding
"strings" in the spec text.

This means that VTTToken can be turned into a "data blob" that is
created by the tokenizer and passed back to the caller.

BUG=319391
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=162548

[jochen (gone - plz use gerrit), 2013-11-22 13:01:03]

lgtm

[commit-bot: I haz the power, 2013-11-22 13:16:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/fs@opera.com/81113003/1

[commit-bot: I haz the power, 2013-11-22 15:31:29]

Change committed as 162548