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This fixes TestTabStatsTracker failures running unit_tests.exe, because
PowerMonitor dchecks if the same observer is added multiple times.

Bug: 956719
Change-Id: Ic01197209d65a7d272f7e943ccc403a1c49107fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618221Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841623}