Make WebVTTParser::collectTimeStamp static

This function is independent of internal WebVTTParser state, while
being used outside of the actual parser.

BUG=305317

Review URL: https://codereview.chromium.org/54463002

git-svn-id: svn://svn.chromium.org/blink/trunk@161059 bbb929c8-8fbe-4397-9dbb-9b2b20218538