Move Singleton and related structs to namespace base

ppapi/proxy/plugin_resource_tracker.h

 // 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.
 
 #ifndef PPAPI_PROXY_PLUGIN_RESOURCE_TRACKER_H_
 #define PPAPI_PROXY_PLUGIN_RESOURCE_TRACKER_H_
 
 #include <map>
 #include <utility>
 
 #include "base/compiler_specific.h"
 #include "base/containers/hash_tables.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_stdint.h"
 #include "ppapi/c/pp_resource.h"
 #include "ppapi/c/pp_var.h"
 #include "ppapi/proxy/ppapi_proxy_export.h"
 #include "ppapi/shared_impl/host_resource.h"
 #include "ppapi/shared_impl/resource_tracker.h"
 
+namespace base {
 template<typename T> struct DefaultSingletonTraits;
+}
 
 namespace ppapi {
 
 namespace proxy {
 
 class PPAPI_PROXY_EXPORT PluginResourceTracker : public ResourceTracker {
  public:
   PluginResourceTracker();
   ~PluginResourceTracker() override;
 
@@ skipping 27 matching lines @@
 
   base::hash_set<PP_Resource> abandoned_resources_;
 
   DISALLOW_COPY_AND_ASSIGN(PluginResourceTracker);
 };
 
 }  // namespace proxy
 }  // namespace ppapi
 
 #endif  // PPAPI_PROXY_PLUGIN_RESOURCE_TRACKER_H_