PPAPI/NaCl: Make NaClIPCAdapter transfer handles more generally

ppapi/proxy/plugin_array_buffer_var.cc

 // Copyright (c) 2012 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 "ppapi/proxy/plugin_array_buffer_var.h"
 
 #include <stdlib.h>
 
 #include <limits>
 
@@ skipping 10 matching lines @@
   if (buffer_.empty())
     return NULL;
   return &(buffer_[0]);
 }
 
 void PluginArrayBufferVar::Unmap() {
   // We don't actually use shared memory yet, so do nothing.
 }
 
 uint32 PluginArrayBufferVar::ByteLength() {
-  return buffer_.size();
+  return static_cast<uint32>(buffer_.size());
 }
 
 }  // namespace ppapi