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This allows DevicesChangedObserver to listen to desired events, instead of being woken up by change of uninterested devices.

BUG=137799
Review URL: https://chromiumcodereview.appspot.com/10836004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149103 0039d316-1c4b-4281-b951-d872f2087c98