Refactors StackingController::GetDefaultParent to ash/wm/common

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index e37ec6c2aa9fb5bb190feb69e8dfe8fad7742d3a..96b6ea2299819fc0790776e21b6cfa4469d016c3 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -95,9 +95,6 @@ class ASH_EXPORT RootWindowController : public ShellObserver {
   // Returns the RootWindowController of the target root window.
   static RootWindowController* ForTargetRootWindow();
 
-  // Returns container which contains a given |window|.
-  static aura::Window* GetContainerForWindow(aura::Window* window);
-
   ~RootWindowController() override;
 
   AshWindowTreeHost* ash_host() { return ash_host_.get(); }