With this change, elements that want to receive viewport rect updates
will need to register on document instead of the browsing context.
This change solves the problem where a browsing context for a document
is guaranteed to exist only while the document is active so browsing
context might not exit by the time DOM node that want to register is
constructed.
This is a part of preparation work before switching to navigables where
this issue becomes more visible.
Having this here instead of in ReplacedBox means we can access it when
figuring out what the "preferred aspect ratio" is.
There's some inconsistency between specs about what this is called, but
they're moving towards referring to this as "natural width/height/
aspect-ratio", so let's copy that terminology.
