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Also update comment in disk_cache::SimpleBackendImpl, and remove some
things that are not used.

Bug: 1007815
Change-Id: I5e239877a52a9906a8e42b62a00a40506e98f89a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288273
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786520}