Move Singleton and related structs to namespace base

extensions/browser/extension_message_filter.cc

Index: extensions/browser/extension_message_filter.cc
diff --git a/extensions/browser/extension_message_filter.cc b/extensions/browser/extension_message_filter.cc
index 053d8eed1890fc7edb012a72fadc2429f11bff4e..c6e2c33cb2d5477121f4ed7fb31524f28427dec5 100644
--- a/extensions/browser/extension_message_filter.cc
+++ b/extensions/browser/extension_message_filter.cc
@@ -29,11 +29,11 @@ class ShutdownNotifierFactory
     : public BrowserContextKeyedServiceShutdownNotifierFactory {
  public:
   static ShutdownNotifierFactory* GetInstance() {
-    return Singleton<ShutdownNotifierFactory>::get();
+    return base::Singleton<ShutdownNotifierFactory>::get();
   }
 
  private:
-  friend struct DefaultSingletonTraits<ShutdownNotifierFactory>;
+  friend struct base::DefaultSingletonTraits<ShutdownNotifierFactory>;
 
   ShutdownNotifierFactory()
       : BrowserContextKeyedServiceShutdownNotifierFactory(