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This CL migrates our Background Fetch layout tests to our WPT suite, and
adds coverage in various new areas, together with a few minor bug fixes.

Tests related to initializing the Service Worker events haven't been
included as the event model has just been updated. New tests will be
added based on that soon.

One file which was removed without being migrated was
"credentials-in-url.https.window.js". This was testing the Fetch API as
opposed to Background Fetch. Instead, the requirement to propagate
exceptions thrown by the Fetch API is tested in fetch.https.window.js.

Bug: 864561
Change-Id: I077be6c5bf8c379e3108f4ec3207011582028477
Reviewed-on: https://chromium-review.googlesource.com/1155118
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579370}