Start/Stop Cards Polling Depending on Notification Center State

Added logic to monitor the state of our notification center checkbox
and to start or stop polling if the checkbox is checked or unchecked
respectively.

BUG=164227
TEST=Get the Notification Center to show up. Click on the Settings Icon (Cog). Uncheck or check the Google Now checkbox. When unchecked, server pings should stop and Chrome should stop running under background mode via the removal of the background permission. When checked, Chrome Now spins back up as long as the user is signed in.

Review URL: https://codereview.chromium.org/37083004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230715