reland of https://codereview.chromium.org/11364053/ w/o removing the per-rootwindow stacking client

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index 5f6945926dd87ea2e3f950dbad5f0d60635345ef..6f3c7f3cfa96a6f05419dc0e5d457e65071f4230 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -21,6 +21,7 @@ namespace aura {
 class RootWindow;
 class Window;
 namespace client {
+class StackingClient;
 class UserActionClient;
 }
 }
@@ -181,6 +182,9 @@ class ASH_EXPORT ShellDelegate {
 
   // Creates a menu model of the context for the |root_window|.
   virtual ui::MenuModel* CreateContextMenu(aura::RootWindow* root_window) = 0;
+
+  // Creates the stacking client. Shell takes ownership of the object.
+  virtual aura::client::StackingClient* CreateStackingClient() = 0;
 };
 
 }  // namespace ash