[cssom-view] Expose inner border dimensions

cf. https://github.com/jquery/jquery/issues/3589#issuecomment-289548410

There seems to be no good method for calculating the dimensions of an element's inner border edge (i.e., content area plus padding plus non-floating scrollbars). One can subtract computed border dimensions from offsetWidth or offsetHeight in the special case of a block-layout HTMLElement, but as far as I can tell it's otherwise impossible.

I think the Element interface should expose something for such calculations, e.g.

• scrollGutterWidth/scrollGutterHeight (the space lost to scrollbars), or
• scrollbarWidth/scrollbarHeight (the scrollbar width), along with a method for identifying non-floating scrollbars, or
• fullClientWidth/fullClientHeight (the dimensions of an inner border edge).

The last one seems most consistent with the rest of CSSOM View.