Convert video-pause* and video-plays* tests to testharness.js

Cleaning up video-pause* and video-plays* 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/2099083002
Cr-Commit-Position: refs/heads/master@{#402085}
6 files changed