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* Previously display idle event will restart ambient mode. Fixed the
  unittest that should have caught this.
* Also added some DVLOG for future debugging.

Bug: None
Change-Id: I49cc798f4d975a3449e506b835469bd740b4308f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354945
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798191}