Wires up modality for aura-mus

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index 9af2e58fd1d6103a213acf093ac55e8ddfb2fe6c..bd8c4fe79056076ef2e6cfc327a3ea560b95bd07 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -247,6 +247,10 @@ class AURA_EXPORT WindowTreeClient
   // Called when the ui::mojom::WindowTree connection is lost, deletes this.
   void OnConnectionLost();
 
+  // Called when a Window property changes. If |key| is handled internally
+  // (maps to a function on WindowTree) returns true.
+  bool HandleInternalPropertyChanged(WindowMus* window, const void* key);
+
   // OnEmbed() calls into this. Exposed as a separate function for testing.
   void OnEmbedImpl(ui::mojom::WindowTree* window_tree,
                    ClientSpecificId client_id,