Drop background color optimization for composited layers

LayoutTests/compositing/squashing/move-squashing-layer-expected.txt

 This tests that squashed layers' offset from renderer is updated properly. If not properly updated, the two divs will appear to be aligned vertically in the pixel results, while really the blue div ('squashed') should be offset 100px left of the green ('host').
 
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 2
         (GraphicsLayer
           (position 8.00 50.00)
           (bounds 100.00 100.00)
           (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #FFFF00)
         )
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 100.00 100.00)
               (bounds 100.00 100.00)
               (contentsOpaque 1)
+              (drawsContent 1)
+              (backgroundColor #008000)
             )
             (GraphicsLayer
               (position 8.00 8.00)
               (bounds 100.00 100.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )