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..2813093ce65a756aeda9184e34d6306ffe444e41 100644
--- a/mojo/public/cpp/bindings/message.h
+++ b/mojo/public/cpp/bindings/message.h
@@ -137,6 +137,10 @@ 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;

[yzshen1, 2016/03/18 23:47:04]

Is this method still needed?

[Anand Mistry (off Chromium), 2016/03/19 01:02:13]

Yes... for unittests. I'll leave it in for now and take a look at the tests
separately. I think we'll want to keep it so we can do tests without them
crashing.

+
+  // DCHECKs if this MessageReceiver is currently bound to a MessagePipe, the
+  // pipe has not been closed, and the pipe has not encountered an error.

[yzshen1, 2016/03/18 23:47:04]

Could you please comment that this may be called from a different thread?

[Anand Mistry (off Chromium), 2016/03/19 01:02:13]

Done.

+  virtual void DCheckValid(const std::string& message) {}
 };
 
 // An alternative to MessageReceiverWithResponder for cases in which it