Add pause_when_not_identical option for ServiceWorkerCacheWriter

This patch adds pause_when_not_identical option for making
ServiceWorkerCacheWriter::MaybeWriteData() return before writing the
new body into the storage when the scripts are not identical.
This option is necessary for extending byte-for-byte update check
of service worker importScripts() resources.

Bug: 648295
Change-Id: If33412cd6e63393ad3fdae514b1588a33f17399a
Reviewed-on: https://chromium-review.googlesource.com/1198651
Commit-Queue: Momoko Hattori <momohatt@google.com>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589492}
5 files changed