Don't force full invalidation on box-sizing change

LayoutTests/fast/repaint/box-sizing-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
-        [1, 50, 314, 200],
-        [1, 50, 314, 200]
+        [400, 0, 40, 140],
+        [380, 0, 20, 140],
+        [0, 100, 440, 40],
+        [0, 80, 440, 20]
       ]
     }
   ]
 }