Fix invalidation during painting of scrollbars

LayoutTests/platform/linux/compositing/squashing/iframe-inside-squashed-layer-expected.txt

 {
   "bounds": [785, 1016],
   "children": [
     {
       "bounds": [785, 1016],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "shouldFlattenTransform": false,
           "children": [
             {
               "position": [8, 8],
               "bounds": [200, 1000],
               "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#D3D3D3"
             },
             {
               "position": [8, 0],
               "bounds": [300, 654],
               "drawsContent": true,
               "repaintRects": [
                 [285, 500, 15, 150],
                 [285, 500, 15, 150],
+                [285, 500, 15, 150],
                 [0, 500, 285, 150],
+                [0, 500, 15, 15],
                 [0, 500, 15, 15]
               ]
             }
           ]
         }
       ]
     }
   ]
 }