Re-land "Drop background color optimization for composited layers"

LayoutTests/compositing/squashing/squashed-layer-loses-graphicslayer-expected.txt

 A squashing RenderLayer that becomes non-composited should correctly send a repaint invalidation to the new container GraphicsLayer that it paints into. When run interactively, hovering over the force-composited gray div should not cause other layers to disappear.
 
 CASE 1, original layer tree:
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
           (children 2
             (GraphicsLayer
               (position 60.00 60.00)
               (bounds 100.00 100.00)
               (contentsOpaque 1)
+              (drawsContent 1)
+              (backgroundColor #808080)
             )
             (GraphicsLayer
               (position 140.00 140.00)
               (bounds 260.00 260.00)
               (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )
 CASE 2, The original composited layer is no longer composited, which then also removes all squashing layers. The important point is that there should be an appropriate repaint to the root GraphicsLayer: