service worker: Add tests for canvas tainting from video.

R=horo

Bug: 780435
Change-Id: Ie8ae9cf8dca55f122a7b4a984ca0a96035b0099f
Reviewed-on: https://chromium-review.googlesource.com/892683
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532814}