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The observer uses a repeated field because a tab or window can be
backgrounded and foregrounded multiple times.

Bug: 921703
Change-Id: If03a3444a237d7c5abf0740a0a3c9c684caea013
Reviewed-on: https://chromium-review.googlesource.com/c/1421681Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626232}