Move Singleton and related structs to namespace base

ui/views/win/windows_session_change_observer.cc

Index: ui/views/win/windows_session_change_observer.cc
diff --git a/ui/views/win/windows_session_change_observer.cc b/ui/views/win/windows_session_change_observer.cc
index c9db4599fe7ffe9bd3066ccf9be96c7598cb1ec0..777bf8c339a5d873a265b72f9e28c2d06edcf11e 100644
--- a/ui/views/win/windows_session_change_observer.cc
+++ b/ui/views/win/windows_session_change_observer.cc
@@ -25,7 +25,7 @@ namespace views {
 class WindowsSessionChangeObserver::WtsRegistrationNotificationManager {
  public:
   static WtsRegistrationNotificationManager* GetInstance() {
-    return Singleton<WtsRegistrationNotificationManager>::get();
+    return base::Singleton<WtsRegistrationNotificationManager>::get();
   }
 
   WtsRegistrationNotificationManager() {
@@ -46,7 +46,8 @@ class WindowsSessionChangeObserver::WtsRegistrationNotificationManager {
   }
 
  private:
-  friend struct DefaultSingletonTraits<WtsRegistrationNotificationManager>;
+  friend struct base::DefaultSingletonTraits<
+      WtsRegistrationNotificationManager>;
 
   void OnWndProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) {
     switch (message) {