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... And make comments in disk_cache.h on ~Backend somewhat reflect
the semantic drift of that API.

It turns out that Blockfile isn't structured in a way that cleanly
permits Close to run after backend destruction --- ~EntryImpl
currently wants to be destroyed on the background thread (due to
using a weak pointer from there), but access to that thread is
owned by backend.

Bug: 811276
Change-Id: Iab92fad91a33dc3d924139bc699671fd21d7a72f
Reviewed-on: https://chromium-review.googlesource.com/999794Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549604}