cc: Make the PictureLayerImpl raster source null until commit.

cc/layers/picture_layer.cc

Index: cc/layers/picture_layer.cc
diff --git a/cc/layers/picture_layer.cc b/cc/layers/picture_layer.cc
index 8a4fd6713c90fb114fab36c20d3abc323230a787..ed18422cc2a0bd81866b101d67c34d0e6e4f47d1 100644
--- a/cc/layers/picture_layer.cc
+++ b/cc/layers/picture_layer.cc
@@ -44,7 +44,7 @@ void PictureLayer::PushPropertiesTo(LayerImpl* base_layer) {
   Layer::PushPropertiesTo(base_layer);
   PictureLayerImpl* layer_impl = static_cast<PictureLayerImpl*>(base_layer);
   // TODO(danakj): Make is_mask_ a constructor parameter for PictureLayer.
-  DCHECK_EQ(layer_impl->is_mask_, is_mask_);
+  DCHECK_EQ(layer_impl->is_mask(), is_mask_);
 
   int source_frame_number = layer_tree_host()->source_frame_number();
   gfx::Size impl_bounds = layer_impl->bounds();