Move enable extensions define to a build flag.

chrome/browser/autocomplete/autocomplete_classifier_factory.cc

Index: chrome/browser/autocomplete/autocomplete_classifier_factory.cc
diff --git a/chrome/browser/autocomplete/autocomplete_classifier_factory.cc b/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
index 0112537e7261435bf08f4744b746dee715cc5a61..cf53be8af346e588a5f38470f471aa481b7c8f73 100644
--- a/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
+++ b/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
@@ -15,8 +15,9 @@
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "components/omnibox/browser/autocomplete_classifier.h"
 #include "components/omnibox/browser/autocomplete_controller.h"
+#include "extensions/features/features.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "extensions/browser/extension_system_provider.h"
 #include "extensions/browser/extensions_browser_client.h"
 #endif
@@ -49,7 +50,7 @@ AutocompleteClassifierFactory::AutocompleteClassifierFactory()
     : BrowserContextKeyedServiceFactory(
         "AutocompleteClassifier",
         BrowserContextDependencyManager::GetInstance()) {
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   DependsOn(
       extensions::ExtensionsBrowserClient::Get()->GetExtensionSystemFactory());
 #endif