Previously there were tests that play audio and tests that play video,
but no tests were playing a file with both audio and video. Also video
was not played to the end. Added PlayVideo test that plays
bear-vp9-opus.webm to the end. Also updated SetBlockMediaLoading tests
to share play_video.html and reduce amount of duplicated code.

Bug: b/153914562
Change-Id: Iba492ec252f692f7aa3f93c319d858a91bb38278
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219404
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773338}