Issue repaints before and after changing the squashing assignment of a RenderLayer.

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 21 matching lines @@
 CASE 2, overlap3 gets added:
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (repaint rects
         (rect 300.00 300.00 100.00 100.00)
+        (rect 300.00 300.00 100.00 100.00)
       )
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 60.00 60.00)
               (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 160.00 160.00 20.00 20.00)
                 (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)
               )
             )
           )
         )
       )
     )
   )
 )
 CASE 3, overlap2 gets removed. Since this does not resize the layer, there should only be a repaint of overlap2:
 (GraphicsLayer
@@ skipping 63 matching lines @@
 CASE 5, overlap2 gets added back:
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (repaint rects
         (rect 220.00 220.00 100.00 100.00)
+        (rect 220.00 220.00 100.00 100.00)
       )
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 60.00 60.00)
               (bounds 400.00 400.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #808080)
             )
             (GraphicsLayer
               (position 220.00 220.00)
               (bounds 180.00 180.00)
               (drawsContent 1)
               (repaint rects
                 (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)
               )
             )
           )
         )
       )
     )
   )
 )
 CASE 6, overlap1 gets added back, and overlap3 gets removed:
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (repaint rects
         (rect 140.00 140.00 100.00 100.00)
+        (rect 140.00 140.00 100.00 100.00)
       )
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 60.00 60.00)
               (bounds 400.00 400.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #808080)
             )
             (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)
+                (rect 0.00 0.00 20.00 20.00)
               )
             )
           )
         )
       )
     )
   )
 )