Add extensions code to use common updater in components/update_client/

extensions/browser/mock_extension_system.cc

 // 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.
 
 #include "extensions/browser/mock_extension_system.h"
 
 #include "extensions/common/extension_set.h"
 
 namespace extensions {
 
@@ skipping 53 matching lines @@
 
 ContentVerifier* MockExtensionSystem::content_verifier() {
   return nullptr;
 }
 
 scoped_ptr<ExtensionSet> MockExtensionSystem::GetDependentExtensions(
     const Extension* extension) {
   return scoped_ptr<ExtensionSet>();
 }
 
+void MockExtensionSystem::InstallUpdate(const std::string& extension_id,
+                                        const base::FilePath& temp_dir) {
+  NOTREACHED();
+}
+
 }  // namespace extensions