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

extensions/shell/browser/shell_extension_system.h

Index: extensions/shell/browser/shell_extension_system.h
diff --git a/extensions/shell/browser/shell_extension_system.h b/extensions/shell/browser/shell_extension_system.h
index f042f2d7ae64081d2fa7fdba4704f7ea6489717b..ca2cb5e8bce9ddfc1c9453a41427d9cc22bb1386 100644
--- a/extensions/shell/browser/shell_extension_system.h
+++ b/extensions/shell/browser/shell_extension_system.h
@@ -64,6 +64,8 @@ class ShellExtensionSystem : public ExtensionSystem {
   ContentVerifier* content_verifier() override;
   scoped_ptr<ExtensionSet> GetDependentExtensions(
       const Extension* extension) override;
+  void InstallUpdate(const std::string& extension_id,
+                     const base::FilePath& temp_dir) override;
 
  private:
   void OnExtensionRegisteredWithRequestContexts(