Move enable extensions define to a build flag.

extensions/browser/extension_registry.h

 // Copyright 2013 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_EXTENSION_REGISTRY_H_
 #define EXTENSIONS_BROWSER_EXTENSION_REGISTRY_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/observer_list.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "extensions/browser/uninstall_reason.h"
 #include "extensions/common/extension_set.h"
+#include "extensions/features/features.h"
 
-#if !defined(ENABLE_EXTENSIONS)
+#if !BUILDFLAG(ENABLE_EXTENSIONS)
 #error "Extensions must be enabled"
 #endif
 
 namespace content {
 class BrowserContext;
 }
 
 namespace extensions {
 class Extension;
 class ExtensionRegistryObserver;
@@ skipping 166 matching lines @@
   base::ObserverList<ExtensionRegistryObserver> observers_;
 
   content::BrowserContext* const browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionRegistry);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_REGISTRY_H_