Simplify VTTParser::collectTimeStamp

Drop the various "is next character and ASCII digit" test that precede
calls to collectDigitsToInt. The latter will perform both the
"out of input" and "is ASCII digit" checks via collectDigits.

BUG=305317

Review URL: https://codereview.chromium.org/104443002

git-svn-id: svn://svn.chromium.org/blink/trunk@163174 bbb929c8-8fbe-4397-9dbb-9b2b20218538