Convert video-source-[load|moved|removed].html tests to testharness.js

Cleaning up video-source-[load|moved|removed].html 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

Review-Url: https://codereview.chromium.org/2116293004
Cr-Commit-Position: refs/heads/master@{#403793}
6 files changed