Move Singleton and related structs to namespace base

chrome/browser/android/offline_pages/offline_page_model_factory.h

Index: chrome/browser/android/offline_pages/offline_page_model_factory.h
diff --git a/chrome/browser/android/offline_pages/offline_page_model_factory.h b/chrome/browser/android/offline_pages/offline_page_model_factory.h
index d92e3665951ac39e6022de5f2156613c11359b0a..25575cf0463dd571dc02bf60f31515f9e2a272da 100644
--- a/chrome/browser/android/offline_pages/offline_page_model_factory.h
+++ b/chrome/browser/android/offline_pages/offline_page_model_factory.h
@@ -8,8 +8,10 @@
 #include "base/macros.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
 
 namespace offline_pages {
 
@@ -23,7 +25,7 @@ class OfflinePageModelFactory : public BrowserContextKeyedServiceFactory {
       content::BrowserContext* context);
 
  private:
-  friend struct DefaultSingletonTraits<OfflinePageModelFactory>;
+  friend struct base::DefaultSingletonTraits<OfflinePageModelFactory>;
 
   OfflinePageModelFactory();
   ~OfflinePageModelFactory() override {}