The WebVTT parsing algorithm allows empty cue text

Adjust tests:
  tc027-empty-cue - PASS -> FAIL, because opposite assumptions
  opera/track/webvtt/parsing:newlines.vtt - one more cue (empty) observed
  opera/interfaces/TextTrackCue/text.html - FAIL -> PASS

(The newlines.vtt file is truncated compared to the
original/upstream version, so chose to update the
expected result rather than modifying the test. Aim to
update the testfile at a later point.)

BUG=305317

Review URL: https://codereview.chromium.org/62833005

git-svn-id: svn://svn.chromium.org/blink/trunk@161924 bbb929c8-8fbe-4397-9dbb-9b2b20218538