[cssom-view] getBoxQuads results for overflow:scroll/auto elements

As we all know, overflow:scroll/auto elements re funky because scrollbars
can be inserted between the padding and the border. This means the content
and padding boxes for such an element are smaller than you'd expect based
on 'width', 'height' and 'padding' properties. For example:
<div style="overflow:scroll; width:200px; height:200px;
padding:10px">...</div>
The children of the <div> will flow into an area that's less than 200px -
10px - 10px wide, if the platform uses scrollbars that need width.

I'm wondering whether to make getBoxQuads subtract scrollbar widths when
computing the size/position of the 'content' and 'padding' boxes for an
overflow:scroll/auto element. Any thoughts?

Rob
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