S13nServiceWorker: Plumb Request#keepalive to FetchEvent.

This makes one of the failing test cases in fetch-event.https.html pass
when run with NetworkService (but there is no expectation change as
FlagExpectations still must expect Failure due to another test case
failure).

Bug: 804878, 695939
Change-Id: I44c152f14282c2cefa056ece13e8f0bf9703676b
Reviewed-on: https://chromium-review.googlesource.com/881382
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531401}
1 file changed