service worker: Refactor and comment WPT test fetch-canvas-tainting.

This uses promise_test for each test case instead of an all-or-nothing
result, which makes the test easier to extend and debug. It also
adds more explanatory comments.

R=horo, shimazu

Bug: 780435
Change-Id: I3ac77448cd19e66b9db806e6b8b5530857713c6c
Reviewed-on: https://chromium-review.googlesource.com/890694
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532339}
5 files changed