Auto merge of #16428 - stshine:box-sizing, r=emilio

layout: Floor to 0 when computing intrinsic sizes with box-sizing:border-box

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Improves bitbucket.

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components/layout/fragment.rs

@@ -937,7 +937,7 @@ impl Fragment {
             }
 
             if self.style.get_position().box_sizing == box_sizing::T::border_box {
-                specified -= border_padding
+                specified = max(Au(0), specified - border_padding);
             }
         }