Convert track-text-track* tests to testharness.js

Cleaning up track-text-track* 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

Review-Url: https://codereview.chromium.org/1952893004
Cr-Commit-Position: refs/heads/master@{#392306}
6 files changed