Refactors StackingController::GetDefaultParent to ash/wm/common

ash/wm/stacking_controller.h

Index: ash/wm/stacking_controller.h
diff --git a/ash/wm/stacking_controller.h b/ash/wm/stacking_controller.h
index 025fc205c99a2f24ea6c2b69e80beb7c08800b8e..4a159301ada343a2f6ae64127a38ca07829a0e42 100644
--- a/ash/wm/stacking_controller.h
+++ b/ash/wm/stacking_controller.h
@@ -24,14 +24,6 @@ class ASH_EXPORT StackingController : public aura::client::WindowTreeClient {
                                  const gfx::Rect& bounds) override;
 
  private:
-  // Returns corresponding system modal container for a modal window.
-  // If screen lock is not active, all system modal windows are placed into the
-  // normal modal container.
-  // Otherwise those that originate from LockScreen container and above are
-  // placed in the screen lock modal container.
-  aura::Window* GetSystemModalContainer(aura::Window* root,
-                                        aura::Window* window) const;
-
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };