Induce lazy background initialization in app_shell

BUG=339267
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/154743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249107 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249107