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This is a follow-up CL for https://crrev.com/c/2114932.
This CL lets FetchRespondWithObserver call methods of FetchEvent
directly.

Bug: 1039558
Change-Id: I332945e4248840cfeddb66ee5599ca18f35394e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352296
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797628}