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This patch adds a new class called ServiceWorkerUpdateChecker and
makes ServiceWorkerRegisterJob use this class. The new class has a
method called Start(), which takes a callback as an argument and
triggers it only when the scripts had any changes to update. Currently
this method runs the callback unconditionally, but following patches
will implement script loading and comparison with the stored resources.

Bug: 648295
Change-Id: Iebbe85b6d5e5cdab27c23873b04811626b5abad0
Reviewed-on: https://chromium-review.googlesource.com/1215458
Commit-Queue: Momoko Hattori <momohatt@google.com>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590260}