Move enable extensions define to a build flag.

extensions/browser/notification_types.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EXTENSIONS_BROWSER_NOTIFICATION_TYPES_H_
 #define EXTENSIONS_BROWSER_NOTIFICATION_TYPES_H_
 
 #include <string>
 
 #include "content/public/browser/notification_types.h"
+#include "extensions/features/features.h"
 
-#if !defined(ENABLE_EXTENSIONS)
+#if !BUILDFLAG(ENABLE_EXTENSIONS)
 #error "Extensions must be enabled"
 #endif
 
 namespace extensions {
 
 // Only notifications fired by the extensions module should be here. The
 // extensions module should not listen to notifications fired by the
 // embedder.
 enum NotificationType {
   // WARNING: This need to match chrome/browser/chrome_notification_types.h.
@@ skipping 172 matching lines @@
                                  const std::string& accelerator);
 
   std::string extension_id;
   std::string command_name;
   std::string accelerator;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_NOTIFICATION_TYPES_H_