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* Screenshot should still work if the power button is pressed first.

Bug: 1024118
Test: Covered by unittests. Manually tested on Slate.
Change-Id: I14c07d0f8419b5755776346d2c5f5e2e156861c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130638
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Kuo Jen Wei <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758723}