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The HostZoomLevelContext class was refcounted, but only ever owned by
a StoragePartitionImpl so remove the refcounting. It also had a custom
deleter doing basically the same work as DeleteOnUIThread so use that
instead. De-virtualize the destructor, too.

Change-Id: I75bf780953e6420e00d488af1560262ca4ed7188
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028934
Auto-Submit: Joshua Bell <jsbell@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743119}