WPT: service worker: canvas tainting with two images from the same URL

This adds a test that does the following:
- Writes to a canvas with a cors same-origin image
- Writes to a canvas with a cors cross-origin image from the same URL
- Tests that the canvas is tainted after the second step.

Bug: 907047
Change-Id: Ie231b442eb9b55c642b3957c065555e6f4997a83
Reviewed-on: https://chromium-review.googlesource.com/c/1347952
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610356}