Revert 114095 - Move the concept of Activation to the Shell.

ui/aura/client/stacking_client.h

Index: ui/aura/client/stacking_client.h
===================================================================
--- ui/aura/client/stacking_client.h	(revision 114098)
+++ ui/aura/client/stacking_client.h	(working copy)
@@ -21,6 +21,14 @@
   // an opportunity to inspect the window and add it to a default parent window
   // of its choosing.
   virtual void AddChildToDefaultParent(Window* window) = 0;
+
+  // Returns true if |window| can be activated or deactivated.
+  // A window manager typically defines some notion of "top level window" that
+  // supports activation/deactivation.
+  virtual bool CanActivateWindow(Window* window) const = 0;
+
+  // Returns the window that should be activated other than |ignore|.
+  virtual Window* GetTopmostWindowToActivate(Window* ignore) const = 0;
 };
 
 }  // namespace aura