Remove dependency on NotificationService for cros a11y settings.

BUG=338387, 268984

Review URL: https://codereview.chromium.org/166273014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253033 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253033