Promoted NullMemoryNotifier outside unit tests.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/130590043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2271 15e8cca8-e42c-11de-a347-f34a4f72eb7d