Fix win64 build failure on size truncation

mojo/public/tests/bindings_connector_unittest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdlib.h>
 #include <string.h>
 
 #include "mojo/public/bindings/lib/bindings_support.h"
 #include "mojo/public/bindings/lib/connector.h"
 #include "mojo/public/bindings/lib/message_queue.h"
@@ skipping 38 matching lines @@
 
   virtual void TearDown() OVERRIDE {
     Close(handle0_);
     Close(handle1_);
   }
 
   void AllocMessage(const char* text, Message* message) {
     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);
   }
 
   void PumpMessages() {
     bindings_support_.Process();
   }
 
  protected:
   Handle handle0_;
@@ skipping 96 matching lines @@
   Close(handle0_);  // Close the handle before writing to it.
 
   bool ok = connector0.Accept(&message);
   EXPECT_FALSE(ok);
 
   EXPECT_TRUE(connector0.EncounteredError());
 }
 
 }  // namespace test
 }  // namespace mojo