Convert track-webvtt-tc[013-015] tests to testharness.js

Cleaning up track-webvtt-tc[013-015] 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/2043513002
Cr-Commit-Position: refs/heads/master@{#397948}
6 files changed