Disable solid background color optimization for composited layers.

LayoutTests/platform/chromium-win/compositing/tiling/huge-layer-with-layer-children-expected.txt

 The yellow box should be large enough to scroll off the bottom. There should be a red box on the first page and a blue box near the bottom of the yellow box. This tests that we can support very large compositing layers with child compositing layers.
 
 This is some text
 (GraphicsLayer
   (bounds 785.00 5116.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 5116.00)
       (contentsOpaque 1)
       (children 1
         (GraphicsLayer
           (position 8.00 72.00)
           (bounds 502.00 5002.00)
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #FFFF00)
           (children 2
             (GraphicsLayer
               (position 51.00 101.00)
               (bounds 200.00 200.00)
               (contentsOpaque 1)
+              (drawsContent 1)
+              (backgroundColor #FF0000)
             )
             (GraphicsLayer
               (position 51.00 4701.00)
               (bounds 200.00 200.00)
               (contentsOpaque 1)
+              (drawsContent 1)
+              (backgroundColor #0000FF)
             )
           )
         )
       )
     )
   )
 )