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Currently Update call will create a new unique pointer and destroy the
existing one. This will cause crash if we get the same object, change
the states and call Update again.

This CL also removes the cache in model, since we should support pass
the same object into Model::Update call.

Bug: 731994
Change-Id: I35ea73bf8b32f7f2a08d2e212dc738cfc4d155d9
Reviewed-on: https://chromium-review.googlesource.com/530484
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478712}