Change message loop to UI.

This is needed in order to receive distributed notifications on Mac, which is how we communicate between the pref pane and the daemon controller.

BUG=293036
R=lambroslambrou@chromium.org, sergeyu@chromium.org

Review URL: https://codereview.chromium.org/26860005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228548 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228548