chromeos: Remove ScreensaverController.

Remove the ScreensaverController class, which isn't being
used anymore. Also remove idle-notification code from
PowerManagerClient.

BUG=none
TBR=yoz@chromium.org

Review URL: https://codereview.chromium.org/26800005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228911 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228911