Removed requirement for ash::Window::transient_parent() presence for system modal dialogs.

BUG=130420
TEST=SystemModalContainerLayoutManagerTest.ModalTransientAndNonTransient

Review URL: https://chromiumcodereview.appspot.com/10514012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@140647 0039d316-1c4b-4281-b951-d872f2087c98
  int mouse_presses() const { return mouse_presses_; }

  int mouse_presses() const { return mouse_presses_; }
