Replace character vectors with StringBuilders in the WebVTT tokenizer

With a goal of simplifying the state management of the
WebVTTTokenizer/WebVTTToken pair, turn all character vectors into
StringBuilders.
Since the tokenizer in general deals with strings, this makes for a
better fit in general (and at least two of the three eliminated FIXMEs
should be able to attest to that).

Also remove the local variable tagName in
WebVTTTreeBuilder::constructTreeFromToken, which appear unused.

BUG=319391

Review URL: https://codereview.chromium.org/75243004

git-svn-id: svn://svn.chromium.org/blink/trunk@162307 bbb929c8-8fbe-4397-9dbb-9b2b20218538