Move enable extensions define to a build flag.

extensions/browser/extension_system.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_EXTENSION_SYSTEM_H_
 #define EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "build/build_config.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "extensions/common/extension.h"
+#include "extensions/features/features.h"
 
-#if !defined(ENABLE_EXTENSIONS)
+#if !BUILDFLAG(ENABLE_EXTENSIONS)
 #error "Extensions must be enabled"
 #endif
 
 class ExtensionService;
 
 #if defined(OS_CHROMEOS)
 namespace chromeos {
 class DeviceLocalAccountManagementPolicyProvider;
 }
 #endif  // defined(OS_CHROMEOS)
@@ skipping 105 matching lines @@
   // temp_dir. Ownership of |temp_dir| in the filesystem is transferred and
   // implementors of this function are responsible for cleaning it up on
   // errors, etc.
   virtual void InstallUpdate(const std::string& extension_id,
                              const base::FilePath& temp_dir) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_SYSTEM_H_