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There were two issues:
* The contentWindow access was throwing an exception because Chrome's
error pages are considered cross-origin.
* The test uses a setTimeout of 10 sec since Mozilla had flaky failures at 5
sec. This issue is still unresolved, see github.com/w3c/ServiceWorker/issues/1194
However, somehow the test seems to pass on local Release despite taking over
6 seconds, so I'm leaving it out of SlowTests for now.

Bug: 758892
Change-Id: I4ff501bcc679db6e7d6ddd7d70adfd33e79bd718
Reviewed-on: https://chromium-review.googlesource.com/664460
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501575}