Fix win64 build failure on size truncation

mojo/public/tests/bindings_connector_unittest.cc

Index: mojo/public/tests/bindings_connector_unittest.cc
===================================================================
--- mojo/public/tests/bindings_connector_unittest.cc	(revision 234554)
+++ mojo/public/tests/bindings_connector_unittest.cc	(working copy)
@@ -57,7 +57,7 @@
     size_t payload_size = strlen(text) + 1;  // Plus null terminator.
     size_t num_bytes = sizeof(MessageHeader) + payload_size;
     message->data = static_cast<MessageData*>(malloc(num_bytes));
-    message->data->header.num_bytes = num_bytes;
+    message->data->header.num_bytes = static_cast<uint32_t>(num_bytes);
     message->data->header.name = 1;
     memcpy(message->data->payload, text, payload_size);
   }