GitLab

Cleaning up video-playback* and sources-fallback* tests
in media/ to use testharness.js instead of video-test.js.
This will enable to upstream these tests to web-platform-tests.

BUG=588956, 443596

Review-Url: https://codereview.chromium.org/2121383005
Cr-Commit-Position: refs/heads/master@{#407143}