Remove position argument from VTTParser::collectTimeStamp

The |position| argument is no longer used by any of the callers.
(They all pass a pointer to a dummy local set to zero.)

BUG=305317

Review URL: https://codereview.chromium.org/134173002

git-svn-id: svn://svn.chromium.org/blink/trunk@164884 bbb929c8-8fbe-4397-9dbb-9b2b20218538