[iOS] Add InMemoryURLIndexFactory::GetDefaultFactory()

ios/chrome/browser/autocomplete/in_memory_url_index_factory.h

Index: ios/chrome/browser/autocomplete/in_memory_url_index_factory.h
diff --git a/ios/chrome/browser/autocomplete/in_memory_url_index_factory.h b/ios/chrome/browser/autocomplete/in_memory_url_index_factory.h
index 03cf9c89b927c4e2b49ffde093f85ab26b219458..7e7cdeddebd6df25840d5c8ecfccfffb0f29a43c 100644
--- a/ios/chrome/browser/autocomplete/in_memory_url_index_factory.h
+++ b/ios/chrome/browser/autocomplete/in_memory_url_index_factory.h
@@ -28,6 +28,11 @@ class InMemoryURLIndexFactory : public BrowserStateKeyedServiceFactory {
       ios::ChromeBrowserState* browser_state);
   static InMemoryURLIndexFactory* GetInstance();
 
+  // Returns the default factory used to build InMemoryURLIndex. Can be
+  // registered with SetTestingFactory to use the InMemoryURLIndex instance
+  // during testing.
+  static TestingFactoryFunction GetDefaultFactory();
+
  private:
   friend struct base::DefaultSingletonTraits<InMemoryURLIndexFactory>;