Move Singleton and related structs to namespace base

chrome/browser/extensions/install_tracker_factory.h

Index: chrome/browser/extensions/install_tracker_factory.h
diff --git a/chrome/browser/extensions/install_tracker_factory.h b/chrome/browser/extensions/install_tracker_factory.h
index 214bc9572731ffa62ca89e311eaa633ff77d2ea3..6a58cf44ea5c7ecf62f0c36a1dd09fc72c85cfc2 100644
--- a/chrome/browser/extensions/install_tracker_factory.h
+++ b/chrome/browser/extensions/install_tracker_factory.h
@@ -7,7 +7,9 @@
 
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace extensions {
 
@@ -19,7 +21,7 @@ class InstallTrackerFactory : public BrowserContextKeyedServiceFactory {
   static InstallTrackerFactory* GetInstance();
 
  private:
-  friend struct DefaultSingletonTraits<InstallTrackerFactory>;
+  friend struct base::DefaultSingletonTraits<InstallTrackerFactory>;
 
   InstallTrackerFactory();
   ~InstallTrackerFactory() override;