fix(mock): Fix unitialized variable in MockWindow.
feat(components): implement css encapsulation for transcluding components

* Extract ComponentCssLoader so it can be shared by ShadowDomComponentFactory and TranscludingComponentFactory
* Provide a built-in implementation of WebPlatformShim using css_shim
* Implement a CSS transformer similar to the one provided by Platform.JS
* Rename EventHandler into ShadowBoundary