NavigableContainer is our home grown concept which already contains the
AOs needed for frame and iframe elements. This patch simply aligns our
HTMLFrameElement implementation with this class.
A couple of notes:
1. The <script> in the <head> element is intentional. The <frameset>
element effectively takes the place of the <body> element, and we
cannot add a <script> to a <frameset> element.
2. We don't render <frameset> or <frame> at all. Rendering is defined
in the following spec:
https://html.spec.whatwg.org/multipage/rendering.html#frames-and-framesets
3. If you load the test page in your browser, you won't see anything,
regardless of (2). Our test infra adds a <pre> element to the "body"
element (which is the <frameset> element here). Such children will
never be rendered. In the future, we could come up with something
better for our test infra to do, but this isn't important anyways
for this test - we can still grab the <pre> element's innerText.
