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

extensions/browser/mock_extension_system.h

Index: extensions/browser/mock_extension_system.h
diff --git a/extensions/browser/mock_extension_system.h b/extensions/browser/mock_extension_system.h
index af847f5e90041dc7ff4bae9ebd65645a0e0aa86d..b055b42ce806754b3f9ab318432758c1dbc9bba3 100644
--- a/extensions/browser/mock_extension_system.h
+++ b/extensions/browser/mock_extension_system.h
@@ -41,6 +41,8 @@ class MockExtensionSystem : 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:
   content::BrowserContext* browser_context_;