Convert track-webvtt* http tests to testharness.js

Cleaning up track-webvtt* http tests 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/2179433002
Cr-Commit-Position: refs/heads/master@{#407163}
4 files changed