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Fills in ServiceWorkerFetchStores.  Adds a stub ServiceWorkerFetchStore class which will need to be filled in. Also adds some tests.

A CL to refactor ServiceWorkerFetchStores -> ServiceWorkerCacheStorage is upcoming.

BUG=392621

Review URL: https://codereview.chromium.org/424863002

Cr-Commit-Position: refs/heads/master@{#288379}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288379 0039d316-1c4b-4281-b951-d872f2087c98