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This CL changes the way that device disabling is expressed in the
DeviceStateRetrievalResponse proto. Instead of being implied when
the |device_state| field is present, it is now explictly set as a
|restore_mode|.

BUG=425574
TEST=Updated unit and browser tests

Review URL: https://codereview.chromium.org/690643002

Cr-Commit-Position: refs/heads/master@{#302426}