Remove state-specific appendTo*/begin* methods from VTTToken

Remove state-specific appendTo*/begin* methods from VTTToken

VTTToken has a number of methods that's only intended for use in
specific states (and for specific token-types). The naming pattern is
appendTo*. After earlier changes, most of these are simple oneliner
wrappers that simply append to the VTTToken::m_data buffer.
Moving towards an implementation that more closely follow (and resemble)
the spec wording, these methods present a problem (and contribute to
making the code unnecessarily complex).

Replace all appendTo*(...) with appendToData(...), and hoist any ASSERTs
into the calling code and simplify them where possible/reasonable.

For appendTo{Class,Annotation}, doing this it becomes apparent that
VTTToken::m_currentBuffer is redundant with m_buffer, so remove the
former in favor of the latter.

BUG=319391
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=162477

[jochen (gone - plz use gerrit), 2013-11-21 13:18:57]

lgtm

[commit-bot: I haz the power, 2013-11-21 13:33:00]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/fs@opera.com/78833002/1

[commit-bot: I haz the power, 2013-11-21 15:37:34]

Change committed as 162477