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

chrome/browser/extensions/test_extension_system.h

Index: chrome/browser/extensions/test_extension_system.h
diff --git a/chrome/browser/extensions/test_extension_system.h b/chrome/browser/extensions/test_extension_system.h
index d0c11e4a54676637fa1d8a6556557cc069f15ac5..9e85ec4fc6162ddf7d13869a5a9437fb3a355f6b 100644
--- a/chrome/browser/extensions/test_extension_system.h
+++ b/chrome/browser/extensions/test_extension_system.h
@@ -58,6 +58,8 @@ class TestExtensionSystem : 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;
 
   // Note that you probably want to use base::RunLoop().RunUntilIdle() right
   // after this to run all the accumulated tasks.