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These notifications are used by the browser to manager power save
blocking, inaccuracies will result in erroneously holding blockers
after playback has stopped.

BUG=43667, 367785
TEST=DCHECKs in WebContentsImpl no longer fire.

Review URL: https://codereview.chromium.org/564943002

Cr-Commit-Position: refs/heads/master@{#294468}