Make mojo::Callback safe to copy across threads.

mojo/public/cpp/bindings/message.h

Index: mojo/public/cpp/bindings/message.h
diff --git a/mojo/public/cpp/bindings/message.h b/mojo/public/cpp/bindings/message.h
index 4b8014a895e5cd954a2c98958e1c1bba3d75f4af..aea5aa7848dd8221e7e23a7edb1135d904ced564 100644
--- a/mojo/public/cpp/bindings/message.h
+++ b/mojo/public/cpp/bindings/message.h
@@ -137,6 +137,11 @@ class MessageReceiverWithStatus : public MessageReceiver {
   // Returns |true| if this MessageReceiver is currently bound to a MessagePipe,
   // the pipe has not been closed, and the pipe has not encountered an error.
   virtual bool IsValid() = 0;
+
+  // DCHECKs if this MessageReceiver is currently bound to a MessagePipe, the
+  // pipe has not been closed, and the pipe has not encountered an error.
+  // This function may be called on any thread.
+  virtual void DCheckValid(const std::string& message) = 0;
 };
 
 // An alternative to MessageReceiverWithResponder for cases in which it