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Currently when screen is off (or Chrome goes to background) for more than 1 min,
peerconnection will be stoppted by PowerMonitor to save battery power. This is
not necesary now since we'll suspend video call when the tab is background. And
users prefer that audio call can continue while they are doing something else.

BUG=513633

Review-Url: https://codereview.chromium.org/2263373003
Cr-Commit-Position: refs/heads/master@{#415372}