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CacheStorageCache dooms entries before closing them if their write operations
fail. This prevents them from being used in the future. In the event that
a ScopedEntry is held by a callback and the callback never gets to run, but
does get deleted, a partially written entry would be closed and will be
reusable.

This is a hypothetical problem. We're not actually sure this can happen since
the CacheStorageCache is kept alive by its manager during async operations. But
for safety, I've made a new smart pointer for disk_cache::Entry*'s that will
by default doom an entry before closing it unless WritingCompleted() has been
called on its deleter. This new smart pointer is used anywhere that Entry*
writing occurs in CacheStorage.

Bug: 617683
Change-Id: Ia5b83bd87b4997fea08cba013fc4701441db6f42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600267
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659210}