Use the correct method for computing a repaint rect that takes into account squashing.

LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt

 Test that layers can be nicely added or removed from a squashed layer, without unnecessary repaints on any layer. Click anywhere to test interactively; keep clicking to proceed through the test.
 
 CASE 1, original layer tree with overlap1 and overlap2:
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
@@ skipping 37 matching lines @@
               (bounds 400.00 400.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #808080)
             )
             (GraphicsLayer
               (position 140.00 140.00)
               (bounds 260.00 260.00)
               (drawsContent 1)
               (repaint rects
+                (rect 140.00 140.00 40.00 40.00)
                 (rect 80.00 80.00 100.00 100.00)
                 (rect 0.00 0.00 100.00 100.00)
                 (rect 0.00 0.00 100.00 100.00)
-                (rect 0.00 0.00 100.00 100.00)
               )
             )
           )
         )
       )
     )
   )
 )
 CASE 3, overlap2 gets removed. Since this does not resize the layer, there should only be a repaint of overlap2:
 (GraphicsLayer
@@ skipping 13 matching lines @@
               (drawsContent 1)
               (backgroundColor #808080)
             )
             (GraphicsLayer
               (position 140.00 140.00)
               (bounds 260.00 260.00)
               (drawsContent 1)
               (repaint rects
                 (rect 160.00 160.00 100.00 100.00)
                 (rect 80.00 80.00 100.00 100.00)
-                (rect 0.00 0.00 100.00 100.00)

[chrishtr, 2014/05/30 16:28:05]

Not sure why this went away exactly, but (80 80 100 100) is the only required
repaint in this case.

               )
             )
           )
         )
       )
     )
   )
 )
 CASE 4, overlap1 gets removed:
 (GraphicsLayer
@@ skipping 87 matching lines @@
             )
             (GraphicsLayer
               (position 140.00 140.00)
               (bounds 180.00 180.00)
               (drawsContent 1)
               (repaint rects
                 (rect 80.00 80.00 100.00 100.00)
                 (rect 0.00 0.00 100.00 100.00)
                 (rect 0.00 0.00 100.00 100.00)
                 (rect 0.00 0.00 100.00 100.00)
+                (rect 0.00 0.00 20.00 20.00)

[chrishtr, 2014/05/30 16:28:05]

This is the amount that overlap1 leaked into the new bounds of
the squashing layer.

               )
             )
           )
         )
       )
     )
   )
 )