CPP bindings: DCHECK when a Callback is destructed without being invoked

mojo/public/cpp/bindings/lib/connector.h

Index: mojo/public/cpp/bindings/lib/connector.h
diff --git a/mojo/public/cpp/bindings/lib/connector.h b/mojo/public/cpp/bindings/lib/connector.h
index 7c1d445a3d6ad7ad8a53e8bb3cc7d9f1d0290279..dfbd514aff20810f4b44dcae77005c2046d51f71 100644
--- a/mojo/public/cpp/bindings/lib/connector.h
+++ b/mojo/public/cpp/bindings/lib/connector.h
@@ -63,6 +63,9 @@ class Connector : public MessageReceiver {
   // a quiescent state.
   ScopedMessagePipeHandle PassMessagePipe();
 
+  // Is the connector bound to a MessagePipe handle?
+  bool is_valid() const { return message_pipe_.is_valid(); }
+
   // Waits for the next message on the pipe, blocking until one arrives or an
   // error happens. Returns |true| if a message has been delivered, |false|
   // otherwise.