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The current code has two bugs.
1. PowerMonitor::RemoveObserver isn't called at destruction. Oops.
2. PowerMonitor::AddObserver is called from a task scheduler
thread, which can race with the power monitor being destructed on
the main thread.

1 is straightforward to fix but 2 isn't really. The power monitor
isn't adding much value, so let's just remove it.

Bug: 912922, 912997
Change-Id: I11cbc028d6ef4a9767096f2553d1cc340aa86e57
Reviewed-on: https://chromium-review.googlesource.com/c/1371390Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615493}