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When a simple disk_cache is in APP_CACHE mode eviction is disabled and
access times do not need to be tracked.  This means that it should be
possible to perform read-only operations without triggering any index
disk writes.

This CL implements this optimization by making the following changes:

* Avoid updating access times in APP_CACHE mode.
* Avoid writing a clean index to disk when closing the backend.
* Avoid dirtying the index when a size update does not make a change.
* Avoid dirtying the index when opening an entry that is in the index.
* Avoid dirtying the index when dooming an entry not in the index.

With these changes its possible to use simple disk_cache to perform
read-only operations in cache_storage without any index writes.

Bug: 905351
Change-Id: I8eed0a72ed166e7b1b3623a7f4c17f7cec7f96d4
Reviewed-on: https://chromium-review.googlesource.com/c/1351539
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612934}