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This hides all notification popups. Its only purpose is for making sure
popups don't interfere with automated tests.

Bug: 807101
Change-Id: I7776af1b0fbd795ed1899a78fa5a7aef1c7b6f9a
Reviewed-on: https://chromium-review.googlesource.com/894152
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533736}