Move Singleton and related structs to namespace base

chrome/browser/apps/per_app_settings_service_factory.h

Index: chrome/browser/apps/per_app_settings_service_factory.h
diff --git a/chrome/browser/apps/per_app_settings_service_factory.h b/chrome/browser/apps/per_app_settings_service_factory.h
index 1111848141fe03fbc93f3a9d6a769efa978c1bf0..23ee62b03938d695808d3e9b32b627319d297a2b 100644
--- a/chrome/browser/apps/per_app_settings_service_factory.h
+++ b/chrome/browser/apps/per_app_settings_service_factory.h
@@ -13,7 +13,10 @@ class BrowserContext;
 
 class PerAppSettingsService;
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
+
 class PerAppSettingsServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
   static PerAppSettingsServiceFactory* GetInstance();
@@ -21,7 +24,7 @@ class PerAppSettingsServiceFactory : public BrowserContextKeyedServiceFactory {
       content::BrowserContext* browser_context);
 
  private:
-  friend struct DefaultSingletonTraits<PerAppSettingsServiceFactory>;
+  friend struct base::DefaultSingletonTraits<PerAppSettingsServiceFactory>;
 
   PerAppSettingsServiceFactory();
   ~PerAppSettingsServiceFactory() override;