ScrollingStateFrameHostingNodes need to store a parent-relative rect, since they take care of the geometry offset between an iframe's renderer and its ancestor scrolling node in the parent document.
ScrollingStateFrameHostingNodes also needs to store the border+padding offset (since that, too, is a property of the parent document, not the content document).
I'll just do this via bug 172917.
*** This bug has been marked as a duplicate of bug 172917 ***
*** This bug has been marked as a duplicate of bug 194002 ***