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ExtensionThrottleEntryInterface was never strictly necessary.
Now that ExtensionThrottleManager does not expose the
ExtensionThrottleEntry to any caller (excluding tests) it
no longer needs to be a reference counted object.

Also, removed the back reference to the ExtensionThrottleManager
in the ExtensionThrottleEntry as that was not necessary.

Bug: none
Change-Id: I5188f7a024304b9159879a9d0763a075c8bfba3c
Reviewed-on: https://chromium-review.googlesource.com/1132353
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577268}