Re-enable the newlines.vtt WebVTT parsing test

With the correct newlines.vtt, the test simply passes.

BUG=318734

Review URL: https://codereview.chromium.org/71593002

git-svn-id: svn://svn.chromium.org/blink/trunk@161957 bbb929c8-8fbe-4397-9dbb-9b2b20218538