Move Singleton and related structs to namespace base

chrome/browser/custom_handlers/protocol_handler_registry_factory.h

Index: chrome/browser/custom_handlers/protocol_handler_registry_factory.h
diff --git a/chrome/browser/custom_handlers/protocol_handler_registry_factory.h b/chrome/browser/custom_handlers/protocol_handler_registry_factory.h
index 2002de38f92f11fcc64ee5ee38b7e1dfb3395018..fc38b59a487b9151b96b9d96eb58a1f9c036bc85 100644
--- a/chrome/browser/custom_handlers/protocol_handler_registry_factory.h
+++ b/chrome/browser/custom_handlers/protocol_handler_registry_factory.h
@@ -11,7 +11,10 @@
 
 class Profile;
 class ProtocolHandlerRegistry;
+
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 // Singleton that owns all ProtocolHandlerRegistrys and associates them with
 // Profiles. Listens for the Profile's destruction notification and cleans up
@@ -35,7 +38,7 @@ class ProtocolHandlerRegistryFactory
   bool ServiceIsNULLWhileTesting() const override;
 
  private:
-  friend struct DefaultSingletonTraits<ProtocolHandlerRegistryFactory>;
+  friend struct base::DefaultSingletonTraits<ProtocolHandlerRegistryFactory>;
 
   ProtocolHandlerRegistryFactory();
   ~ProtocolHandlerRegistryFactory() override;