Cache the Document title

[nox]

Whenever Document.title_changed() is called by the virtual methods on HTMLTitleElement, the tree is traversed to find the first title element in the document and notify the chrome of the change.

The current title element should be cached in Document in a MutNullableHeap<JS<HTMLTitleElement>> field and be updated in bind_from_tree() and unbind_from_tree() in the virtual methods on HTMLTitleElement.

[nox]

Cc'ing @nick-thompson who said he was interested in looking into this.

[nick-thompson]

Thanks, I'm on it!

[frewsxcv]

...and be updated in bind_from_tree() and unbind_from_tree() in the virtual methods on HTMLTitleElement

Sinmce an SVG <title> element is (presumably) not an HTMLTitleElement, how will this work in the SVG case?