Convert track tests from video-test.js to testharness.js based
Cleaning up text-css-all*, track-css-cue* and track-css-matching* tests
in media/track to use testharness.js instead of video-test.js.
This will enable to upstream these tests to web-platform-tests.
BUG=
588956
Committed:
https://crrev.com/c8cf76ce31a999ccd27562c753260101075df9ce
Cr-Commit-Position: refs/heads/master@{#386660}