Add ServiceWorkerSingleScriptUpdateChecker

This patch adds a new class called ServiceWorkerSingleScriptUpdateChecker,
and makes ServiceWorkerUpdateChecker call this class. This new class
currently calls UpdateChecker's method unconditionally, but following
patches will implement these to SWSingleScriptUpdateChecker:
* collect stored scripts which are used in the version to update
* load the counterpart of the stored scripts from network
* compare the stored scripts and the loaded scripts one by one
* when detected any changes, pause the comparison and stop the entire update check

Bug: 648295
Change-Id: I23835fa62838b9c1a979e7ae96f296de6b6ff074
Reviewed-on: https://chromium-review.googlesource.com/1220854
Commit-Queue: Momoko Hattori <momohatt@google.com>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590979}
6 files changed