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Added unit tests to TabUsageRecorder with mocking of application state

Continuation of http://crrev/c/802464 to test that Active, Inactive,
and Background application states are interpreted correctly.

Bug: 781361
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ie86cfca571e8e0a4159a591d0d357a3458384483
Reviewed-on: https://chromium-review.googlesource.com/802618Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521453}