Listen for APP_TERMINATING in notification ui; close windows earlier in the process before they get clobbered by browser_shutdown leading to a potential double-close.

      BUG=58053
TEST=exit chrome with notifications open

Review URL: http://codereview.chromium.org/5319011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67883 0039d316-1c4b-4281-b951-d872f2087c98