HTMLElement::Offset{Parent,Left,Right} should call HTMLBodyElement::is_the_html_body_element

[frewsxcv]

https://github.com/servo/servo/pull/10513

Spec links:

• https://drafts.csswg.org/cssom-view/#dom-htmlelement-offsetparent
• https://drafts.csswg.org/cssom-view/#dom-htmlelement-offsettop
• https://drafts.csswg.org/cssom-view/#dom-htmlelement-offsetleft

Relevant code

[frewsxcv]

Ideally there'd be a regression test for this, but I don't know how difficult that would be to write.

[jdm]

There's a solution in #13659; it's just missing a regression test and needs to be rebased.

[nox]

Still to be done.