Move Singleton and related structs to namespace base

ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h

Index: ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h
diff --git a/ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h b/ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h
index 93c75ccc1ed6c80b3809c665ed54422c89a144d5..89acfc5cc22758e8aaff82af88042b864c01f91c 100644
--- a/ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h
+++ b/ios/chrome/browser/autocomplete/autocomplete_classifier_factory.h
@@ -9,8 +9,11 @@
 #include "base/memory/scoped_ptr.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
 
+namespace base {
 template <typename T>
 struct DefaultSingletonTraits;
+}  // namespace base
+
 class AutocompleteClassifier;
 
 namespace ios {
@@ -31,7 +34,7 @@ class AutocompleteClassifierFactory : public BrowserStateKeyedServiceFactory {
   static TestingFactoryFunction GetDefaultFactory();
 
  private:
-  friend struct DefaultSingletonTraits<AutocompleteClassifierFactory>;
+  friend struct base::DefaultSingletonTraits<AutocompleteClassifierFactory>;
 
   AutocompleteClassifierFactory();
   ~AutocompleteClassifierFactory() override;