Absolutes are not shrinking to fit properly in Acid2

[pcwalton]

Line 2 blockquote problem in Acid2.

/cc @pradeep90

[pradeep90]

@pcwalton I believe it is because of #1708
In bubble_width, calculation of pref width and min width is wrong (because it adds in margins and stuff).

Box::guess_width gives up

width + margin_left + margin_right + padding_left + padding_right + self.border.get().left + self.border.get().right

as the value for the content width o_0

So, for the blockquote in acid2.html, we get shrink-to-fit width as width of float kid + our margin + our border

@SimonSapin suggested using http://dev.w3.org/csswg/css-sizing/#max-content-measuremax-measuremax-content-size as reference for the min and pref width calculation because it is more precise than the CSS 2.1 reference.

[pcwalton]

Fixed.