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This CL migrates the backend of ServiceWorkerDiskCache from Default to
SimpleCache on all platforms.

If old DiskCache images exist,
(1) DiskCache backend fails to open an pre-existing data due to incompatible
format (in both Default->SimpleCache and SimpleCache->Default cases),
(2) ServiceWorkerStorage wipes out all cached scripts and registrations via
DeleteAndStartOver, and
(3) navigation falls back to network.

BUG=487482
TEST=manual (use DiskCache and SimpleCache alternately and make sure the system can recover)

Review URL: https://codereview.chromium.org/1140853002

Cr-Commit-Position: refs/heads/master@{#330274}