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This CL refactors WorkerWatcher and adds a ServiceWorkerClient
class that will facilitate adding support for service worker
clients in the graph.

This CL was split from
https://chromium-review.googlesource.com/c/chromium/src/+/2355089,
which had to be reverted because of crashes on canary.

Bug: 993029
Change-Id: Ia2386a8611547dbd9a629ed18fdbee67a8d942aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411652Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807628}