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

LayoutTests/compositing/squashing/squash-above-fixed-1-expected.txt

 The gray div is a composited fixed-position element, and the cyan/lime elements should be squashed together on top. When scrolling, paragraphs may pop in-and-out of the squashing layer when they change overlapping status with respect to the composited layer underneath.
 
 This scenario tests (1) that content repaints correctly as layers pop in and out of squashing, and (2) that the positioning of the squashing layer remains correct (i.e. scrolls properly) when the squashing layer is on top of a fixed-position composited layer.
 
 CASE 1, original layer tree:
 (GraphicsLayer
   (bounds 785.00 1400.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 1400.00)
@@ skipping 29 matching lines @@
 )
 CASE 2, scrolling y to 80, new layers will be squashed, so things repaint:
 (GraphicsLayer
   (bounds 785.00 1400.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 1400.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (repaint rects
+        (rect 0.00 500.00 200.00 100.00)
+        (rect 0.00 400.00 200.00 100.00)
+        (rect 0.00 300.00 200.00 100.00)

[chrishtr, 2014/06/02 20:16:24]

These repaints are because the layer they were squashing into became no longer
composited (because it no longer overlaps the fixed pos element).

         (rect 0.00 200.00 200.00 100.00)
         (rect 0.00 100.00 200.00 100.00)
       )
       (children 2
         (GraphicsLayer
           (position 100.00 230.00)
           (bounds 300.00 300.00)
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #D3D3D3)
@@ skipping 56 matching lines @@
 )
 CASE 4, scrolling y to 170 new layers will be squashed, so things repaint:
 (GraphicsLayer
   (bounds 785.00 1400.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 1400.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (repaint rects
+        (rect 0.00 600.00 200.00 100.00)
+        (rect 0.00 500.00 200.00 100.00)
+        (rect 0.00 400.00 200.00 100.00)

[chrishtr, 2014/06/02 20:16:24]

Similar situation to above.

         (rect 0.00 300.00 200.00 100.00)
         (rect 0.00 200.00 200.00 100.00)
       )
       (children 2
         (GraphicsLayer
           (position 100.00 320.00)
           (bounds 300.00 300.00)
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #D3D3D3)
@@ skipping 12 matching lines @@
               (bounds 200.00 300.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )