Move Singleton and related structs to namespace base

ui/views/accessibility/ax_aura_obj_cache.h

Index: ui/views/accessibility/ax_aura_obj_cache.h
diff --git a/ui/views/accessibility/ax_aura_obj_cache.h b/ui/views/accessibility/ax_aura_obj_cache.h
index 81384a177d93d4efe6d849438f65f43af37b4ff5..95c46f387104a498287303f6c2689f05a71d510d 100644
--- a/ui/views/accessibility/ax_aura_obj_cache.h
+++ b/ui/views/accessibility/ax_aura_obj_cache.h
@@ -11,7 +11,9 @@
 #include "base/basictypes.h"
 #include "ui/views/views_export.h"
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace aura {
 class Window;
@@ -63,7 +65,7 @@ class VIEWS_EXPORT AXAuraObjCache {
   bool is_destroying() { return is_destroying_; }
 
  private:
-  friend struct DefaultSingletonTraits<AXAuraObjCache>;
+  friend struct base::DefaultSingletonTraits<AXAuraObjCache>;
 
   AXAuraObjCache();
   virtual ~AXAuraObjCache();