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

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

Index: LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
diff --git a/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt b/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
index e53127f768710359e6ef550533733d1f870afb9c..88653bed51b3c001ba047ab2d895d1208899aa96 100644
--- a/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
+++ b/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
@@ -55,10 +55,10 @@ CASE 2, overlap3 gets added:
               (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)
               )
             )
           )
@@ -92,7 +92,6 @@ CASE 3, overlap2 gets removed. Since this does not resize the layer, there shoul
               (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)
               )
             )
           )
@@ -200,6 +199,7 @@ CASE 6, overlap1 gets added back, and overlap3 gets removed:
                 (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)
               )
             )
           )