GTK: Stop listening to gtk signals in the omnibox before destroying the model.

      BUG=123530
TEST=none


Review URL: http://codereview.chromium.org/10103012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@132498 0039d316-1c4b-4281-b951-d872f2087c98