Move enable extensions define to a build flag.

chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc

Index: chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc
diff --git a/chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc b/chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc
index b997043d8734436251981d81efdbe525fe1dcd05..af917eb87a32388658b32701692d36a8422741a8 100644
--- a/chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc
+++ b/chrome/browser/autocomplete/chrome_autocomplete_provider_client.cc
@@ -32,8 +32,9 @@
 #include "components/prefs/pref_service.h"
 #include "components/sync/driver/sync_service_utils.h"
 #include "content/public/browser/notification_service.h"
+#include "extensions/features/features.h"
 
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
 #include "chrome/browser/autocomplete/keyword_extensions_delegate_impl.h"
 #endif
 
@@ -142,7 +143,7 @@ ChromeAutocompleteProviderClient::GetShortcutsBackendIfExists() {
 std::unique_ptr<KeywordExtensionsDelegate>
 ChromeAutocompleteProviderClient::GetKeywordExtensionsDelegate(
     KeywordProvider* keyword_provider) {
-#if defined(ENABLE_EXTENSIONS)
+#if BUILDFLAG(ENABLE_EXTENSIONS)
   return base::MakeUnique<KeywordExtensionsDelegateImpl>(profile_,
                                                          keyword_provider);
 #else