Refactors StackingController::GetDefaultParent to ash/wm/common

ash/wm/aura/wm_window_aura.h

Index: ash/wm/aura/wm_window_aura.h
diff --git a/ash/wm/aura/wm_window_aura.h b/ash/wm/aura/wm_window_aura.h
index 5dd2266ea06602ef0ba54219a978498d12755ae8..90e1453b7922b142102ec3aecada9b793b5667b2 100644
--- a/ash/wm/aura/wm_window_aura.h
+++ b/ash/wm/aura/wm_window_aura.h
@@ -60,6 +60,7 @@ class ASH_EXPORT WmWindowAura : public WmWindow, public aura::WindowObserver {
   gfx::Size GetMaximumSize() const override;
   bool GetTargetVisibility() const override;
   bool IsVisible() const override;
+  bool IsSystemModal() const override;
   bool GetBoolProperty(WmWindowProperty key) override;
   int GetIntProperty(WmWindowProperty key) override;
   const WindowState* GetWindowState() const override;