Fix win64 build failure on size truncation

mojo/system/message_pipe_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 "mojo/system/message_pipe.h"
 
 #include "base/memory/ref_counted.h"
 #include "base/threading/platform_thread.h"  // For |Sleep()|.
 #include "base/time/time.h"
 #include "mojo/system/waiter.h"
@@ skipping 196 matching lines @@
 }
 
 TEST(MessagePipeTest, CloseWithQueuedIncomingMessages) {
   scoped_refptr<MessagePipe> mp(new MessagePipe());
 
   int32_t buffer[1];
   const uint32_t kBufferSize = static_cast<uint32_t>(sizeof(buffer));
   uint32_t buffer_size;
 
   // Write some messages from port 1 (to port 0).
-  for (size_t i = 0; i < 5; i++) {
+  for (int32_t i = 0; i < 5; i++) {
     buffer[0] = i;
     EXPECT_EQ(MOJO_RESULT_OK,
               mp->WriteMessage(1,
                                buffer, kBufferSize,
                                NULL,
                                MOJO_WRITE_MESSAGE_FLAG_NONE));
   }
 
   // Port 0 shouldn't be empty.
   buffer_size = 0;
@@ skipping 279 matching lines @@
 
     mp->CancelAllWaiters(1);
     mp->Close(1);
   }  // Joins |thread|.
   EXPECT_EQ(MOJO_RESULT_FAILED_PRECONDITION, result);
 }
 
 }  // namespace
 }  // namespace system
 }  // namespace mojo