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This is consistent with public/platform/modules/cache_storage and
content/*/cache_storage.

Change-Id: Idb67f16bccfce3f035a9375e10a5ea5f4a99647f
Reviewed-on: https://chromium-review.googlesource.com/1028998Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553858}