Mojo C++ bindings: introduce an optional array to store transferred interface IDs in messages.

mojo/public/cpp/bindings/associated_group_controller.h

Index: mojo/public/cpp/bindings/associated_group_controller.h
diff --git a/mojo/public/cpp/bindings/associated_group_controller.h b/mojo/public/cpp/bindings/associated_group_controller.h
index 6ca94f950e032042a27e59ba5e83edef5a447a0b..e42a8895d269dc3e896aecd18f2dad8acfb5e967 100644
--- a/mojo/public/cpp/bindings/associated_group_controller.h
+++ b/mojo/public/cpp/bindings/associated_group_controller.h
@@ -38,6 +38,9 @@ class MOJO_CPP_BINDINGS_EXPORT AssociatedGroupController
   // Typically, this method is used to (1) create an endpoint handle for the
   // master interface; or (2) create an endpoint handle on receiving an
   // interface ID from the message pipe.
+  //
+  // On failure, the method returns an invalid handle. Usually that is because
+  // the ID has already been used to create a handle.
   virtual ScopedInterfaceEndpointHandle CreateLocalEndpointHandle(
       InterfaceId id) = 0;