Convert webvtt regions tests from video-test.js to testharness.js based

Cleaning up tests in media/track/regions-webvtt 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/1854553003

Cr-Commit-Position: refs/heads/master@{#385722}
12 files changed