Update TextTrackCue tests to use VTTCue where appropriate

The Opera tests were fixed upstream and re-imported:
https://github.com/w3c/web-platform-tests/commit/8c08468d5494c9026c2a857f366c631651c413fd
https://github.com/w3c/web-platform-tests/commit/da34397c2c16170da60e5c7638e00c0edadcf0be

A new test in cues.html requres a more recent testharness.js
than we have and was disabled.

BUG=270340

Review URL: https://codereview.chromium.org/72543003

git-svn-id: svn://svn.chromium.org/blink/trunk@162026 bbb929c8-8fbe-4397-9dbb-9b2b20218538