Move enable extensions define to a build flag.

extensions/common/extension.h

 // Copyright (c) 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_COMMON_EXTENSION_H_
 #define EXTENSIONS_COMMON_EXTENSION_H_
 
 #include <map>
 #include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/linked_ptr.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/thread_checker.h"
 #include "extensions/common/extension_id.h"
 #include "extensions/common/extension_resource.h"
 #include "extensions/common/install_warning.h"
 #include "extensions/common/manifest.h"
 #include "extensions/common/url_pattern_set.h"
+#include "extensions/features/features.h"
 #include "url/gurl.h"
 
-#if !defined(ENABLE_EXTENSIONS)
+#if !BUILDFLAG(ENABLE_EXTENSIONS)
 #error "Extensions must be enabled"
 #endif
 
 namespace base {
 class DictionaryValue;
 class Version;
 }
 
 namespace extensions {
 class PermissionSet;
@@ skipping 521 matching lines @@
   const PermissionSet& permissions;
 
   UpdatedExtensionPermissionsInfo(const Extension* extension,
                                   const PermissionSet& permissions,
                                   Reason reason);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_EXTENSION_H_