Allow to have a transparent UA dependent background.

cc/layers/picture_layer_impl.h

Index: cc/layers/picture_layer_impl.h
diff --git a/cc/layers/picture_layer_impl.h b/cc/layers/picture_layer_impl.h
index 03316dcd011b39dcfee78187d45dc73035c2c2a7..c7fa848f86c43e0c8fd4e3891fa986811f52854c 100644
--- a/cc/layers/picture_layer_impl.h
+++ b/cc/layers/picture_layer_impl.h
@@ -156,6 +156,8 @@ class CC_EXPORT PictureLayerImpl
 
   bool nearest_neighbor_;
 
+  bool has_alpha_channel_;
+
   // Use this instead of |visible_layer_rect()| for tiling calculations. This
   // takes external viewport and transform for tile priority into account.
   gfx::Rect viewport_rect_for_tile_priority_in_content_space_;