Handle empty lines in the 'TimingsAndSettings' state in the WebVTTParser

As per the WebVTT parsing specification, step 27 ("If line is the empty
string, then discard cue and jump to the step labeled cue loop."), the
cue should be discarded and the cue loop restarted if an "identifier"
line is followed by an empty line. Transitioning to the 'BadCue' state
will lose the transition, so we need to transition explicitly from the
'TimingsAndSettings' state (to the 'Id' state.)

BUG=314056

Review URL: https://codereview.chromium.org/55853002

git-svn-id: svn://svn.chromium.org/blink/trunk@161184 bbb929c8-8fbe-4397-9dbb-9b2b20218538