Fix win64 build failure on size truncation

mojo/services/native_viewport/native_viewport_controller.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/services/native_viewport/native_viewport_controller.h"
 
+#include <limits>
+
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/stringprintf.h"
 #include "gpu/command_buffer/client/gl_in_process_context.h"
 #include "gpu/command_buffer/client/gles2_implementation.h"
 #include "mojo/services/native_viewport/native_viewport.h"
 #include "ui/events/event.h"
 
 namespace mojo {
 namespace services {
@@ skipping 46 matching lines @@
   SendString(base::StringPrintf("Sized to: %d x %d",
                                 size.width(),
                                 size.height()));
 }
 
 void NativeViewportController::OnDestroyed() {
   base::MessageLoop::current()->Quit();
 }
 
 void NativeViewportController::SendString(const std::string& string) {
-  WriteMessage(pipe_, string.c_str(), string.size()+1, NULL, 0,
-                MOJO_WRITE_MESSAGE_FLAG_NONE);
+  DCHECK_LT(string.size() + 1, std::numeric_limits<uint32_t>::max());
+  WriteMessage(pipe_, string.c_str(), static_cast<uint32_t>(string.size() + 1),
+               NULL, 0, MOJO_WRITE_MESSAGE_FLAG_NONE);
 }
 
 }  // namespace services
 }  // namespace mojo