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Historically the simple disk_cache backend has assumed all accesses
occurred on the browser IO thread.  In order to support running
cache_storage on a separate sequence this CL removes the IO thread
assumptions from simple disk_cache.

Bug: 960012
Change-Id: Idde0f6049aad403318d32b65a160515ee5d67fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643234Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665990}