Mojo C++ bindings: remove some usage of AssociatedGroup from user code.

services/ui/ws/window_tree_binding.cc

Index: services/ui/ws/window_tree_binding.cc
diff --git a/services/ui/ws/window_tree_binding.cc b/services/ui/ws/window_tree_binding.cc
index 97c3953da84dc11f6c06d4dce6a082a1eb69189f..e1564acc4c5d80ba1735f6a648a2dc0dc5b774b3 100644
--- a/services/ui/ws/window_tree_binding.cc
+++ b/services/ui/ws/window_tree_binding.cc
@@ -49,8 +49,7 @@ mojom::WindowTreePtr DefaultWindowTreeBinding::CreateInterfacePtrAndBind() {
 }
 
 mojom::WindowManager* DefaultWindowTreeBinding::GetWindowManager() {
-  client_->GetWindowManager(
-      MakeRequest(&window_manager_internal_, client_.associated_group()));
+  client_->GetWindowManager(MakeRequest(&window_manager_internal_));
   return window_manager_internal_.get();
 }