Move Singleton and related structs to namespace base

ui/base/x/x11_foreign_window_manager.h

Index: ui/base/x/x11_foreign_window_manager.h
diff --git a/ui/base/x/x11_foreign_window_manager.h b/ui/base/x/x11_foreign_window_manager.h
index 354cfba6d82be98c068a1dd88ebf48d2e3edc73f..a9167b15598c5d41980515e6568b6c09f59be28e 100644
--- a/ui/base/x/x11_foreign_window_manager.h
+++ b/ui/base/x/x11_foreign_window_manager.h
@@ -15,7 +15,9 @@
 
 // A process wide singleton for selecting events on X windows which were not
 // created by Chrome.
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace ui {
 
@@ -39,7 +41,7 @@ class UI_BASE_EXPORT XForeignWindowManager {
   void OnWindowDestroyed(XID xid);
 
  private:
-  friend struct DefaultSingletonTraits<XForeignWindowManager>;
+  friend struct base::DefaultSingletonTraits<XForeignWindowManager>;
 
   struct Request {
     Request(int request_id, long entry_event_mask);