Use PositionedMovementLayout for changes to positioned elements' margins

LayoutTests/fast/repaint/relative-margin-change-repaint-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
-        [8, 8, 30, 30]
+        [0, 60, 200, 400],
+        [0, 60, 100, 100],
+        [0, 40, 200, 400],
+        [0, 40, 100, 100],
+        [0, 20, 800, 400],
+        [0, 0, 800, 400]
       ]
     }
   ]
 }