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

LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt

Index: LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt
diff --git a/LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt b/LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt
index 187b2e246ffdd9d95b5b9f40d48097dbb1eeba1b..b7f10c5694302eeee76ed808a36a8c27a1ce5e36 100644
--- a/LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt
+++ b/LayoutTests/compositing/iframes/iframe-size-from-zero-expected.txt
@@ -31,6 +31,8 @@
                                   (position 18.00 10.00)
                                   (bounds 210.00 210.00)
                                   (contentsOpaque 1)
+                                  (drawsContent 1)
+                                  (backgroundColor #0000FF)
                                 )
                               )
                             )
@@ -52,6 +54,8 @@
         (GraphicsLayer
           (position 5.00 5.00)
           (bounds 50.00 50.00)
+          (drawsContent 1)
+          (backgroundColor #00000033)
         )
       )
     )