Defer setting type in the WebVTT tokenizer until emitting the token

Remove setting of the token type from begin* (and analog) methods
in VTTToken, and add a new setType. Call setType from the
token-emitting methods in VTTTokenizer. Remove ASSERTs that no
longer apply.

Because of this change, VTTTokenizer::haveBufferedCharacterToken
can no longer return a value based on the type of token - make it return
false always (there should never be a buffered character token - a token
should be emitted after each and every call to nextToken that returns
true). This also means that "EOF" (read: end-of-string) handling needs to
be improved. Adopt the method from the HTML parser, that appends a segment
with an EOF marker (a NUL) to the input, and adjust EOF handling to match
(make sure to consume the EOF, and exit early and return false if
encountering a EOF mark at the start of the FSM). While doing this,
also hide the "implementation detail" that SegmentedString is used, by
just passing a String to the tokenizer via the constructor.

After doing the above, it becomes apparent that the EndTagOpenState is
redundant with the EndTagState, so they can be merged. (The former state
does not appear in the spec text.)

A number of end-of-input cases are fixed:

tags.html - "<c." now parses correctly.
timestamp.html - "<00:00:00.500" now parses correctly.

New tests:

entities.html - (A number of FAIL -> PASS transitions here compared to
               previously - due to actually setting the correct token-
               type for content with only an entity (or something looking
               like an entity. Also no longer triggers an assert in Debug.)

BUG=319391

Review URL: https://codereview.chromium.org/77553004

git-svn-id: svn://svn.chromium.org/blink/trunk@162373 bbb929c8-8fbe-4397-9dbb-9b2b20218538