Mojo C++ bindings: support disconnect with a reason.

mojo/public/cpp/bindings/lib/interface_endpoint_client.cc

Index: mojo/public/cpp/bindings/lib/interface_endpoint_client.cc
diff --git a/mojo/public/cpp/bindings/lib/interface_endpoint_client.cc b/mojo/public/cpp/bindings/lib/interface_endpoint_client.cc
index 0f15398545073e92465b29a7bf217756288238a9..368a521612be364cd18f71bc0f54b4aaea278252 100644
--- a/mojo/public/cpp/bindings/lib/interface_endpoint_client.cc
+++ b/mojo/public/cpp/bindings/lib/interface_endpoint_client.cc
@@ -281,8 +281,16 @@ void InterfaceEndpointClient::NotifyError() {
 
   control_message_proxy_.OnConnectionError();
 
-  if (!error_handler_.is_null())
+  if (!error_handler_.is_null()) {
     error_handler_.Run();
+  } else if (!error_with_reason_handler_.is_null()) {
+    // Make a copy on the stack. If we directly pass a reference to a member of
+    // |control_message_handler_|, that reference will be invalidated as soon as
+    // the user destroys the interface endpoint.
+    std::string description = control_message_handler_.disconnect_description();
+    error_with_reason_handler_.Run(
+        control_message_handler_.disconnect_custom_reason(), description);
+  }
 }
 
 bool InterfaceEndpointClient::HandleValidatedMessage(Message* message) {