Change MockServiceWorkerResponseWriter always async

This is a preparation for using mojo version of
ServiceWorkerResourceWriter. The mojo version is always async
so it doesn't make much sense to support sync callbacks in
tests.

Bug: 1055677
Change-Id: I7ebcca7b6546fbbe288c4315d7f4e5fa9ce6f900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2269378
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783422}