Move Singleton and related structs to namespace base

ui/views/focus/widget_focus_manager.h

Index: ui/views/focus/widget_focus_manager.h
diff --git a/ui/views/focus/widget_focus_manager.h b/ui/views/focus/widget_focus_manager.h
index f87814a3047fe9e5b7465f42047434cea7f02265..1a25b6d85d4f5d01a3f371b40b6730c7013df477 100644
--- a/ui/views/focus/widget_focus_manager.h
+++ b/ui/views/focus/widget_focus_manager.h
@@ -10,7 +10,9 @@
 #include "ui/gfx/native_widget_types.h"
 #include "ui/views/views_export.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace views {
 
@@ -48,7 +50,7 @@ class VIEWS_EXPORT WidgetFocusManager {
   void DisableNotifications() { enabled_ = false; }
 
  private:
-  friend struct DefaultSingletonTraits<WidgetFocusManager>;
+  friend struct base::DefaultSingletonTraits<WidgetFocusManager>;
 
   WidgetFocusManager();
   ~WidgetFocusManager();