Allow controls other than windows to send alert accessibility events.

This makes battery alerts and network state change notifications emit
spoken (and braille) feedback again and fixes a crash on ash debug
builds when accessibility is enabled.

BUG=310766
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/74083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235764 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235764