shadow-dom: Refine a test for Window object's named properties.

This change updates an existing test that checks whether an element in a shadow
tree is exposed to window object's named properties. The test is split into
three files, each of which tests:

* Child browsing context (e.g. <frame> and <iframe>) that has browsing context
  name;
* Elements with name attribute; and
* Elements with id attribute.

This classification is based on the search order of Window object's named
properties (see:
<http://www.whatwg.org/specs/web-apps/current-work/multipage/browsers.html#named-access-on-the-window-object>).