Revert 189518 "PPAPI: Remove threading options; it's always on"

trunk/src/ppapi/proxy/plugin_resource_tracker.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_resource_tracker.h"
 
 #include "base/logging.h"
 #include "base/memory/singleton.h"
 #include "ppapi/proxy/plugin_dispatcher.h"
 #include "ppapi/proxy/plugin_globals.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/serialized_var.h"
 #include "ppapi/shared_impl/proxy_lock.h"
 #include "ppapi/shared_impl/resource.h"
 #include "ppapi/shared_impl/var.h"
 
 namespace ppapi {
 namespace proxy {
 
-PluginResourceTracker::PluginResourceTracker() : ResourceTracker(THREAD_SAFE) {
+PluginResourceTracker::PluginResourceTracker() {
 }
 
 PluginResourceTracker::~PluginResourceTracker() {
 }
 
 PP_Resource PluginResourceTracker::PluginResourceForHostResource(
     const HostResource& resource) const {
   HostResourceMap::const_iterator found = host_resource_map_.find(resource);
   if (found == host_resource_map_.end())
     return 0;
@@ skipping 31 matching lines @@
       // the instance was destroyed. In that case the browser-side resource has
       // already been freed correctly on the browser side.
       dispatcher->Send(new PpapiHostMsg_PPBCore_ReleaseResource(
           API_ID_PPB_CORE, object->host_resource()));
     }
   }
 }
 
 }  // namespace proxy
 }  // namespace ppapi