cc/blimp: Eliminate Layer updates for common cases.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 517a1c6a634511b8774f01d285dcac06f1038d5d..1d53e26d30301008dbd46ad4b5421e03e1a2f58b 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -420,6 +420,9 @@ void Layer::SetMaskLayer(Layer* mask_layer) {
   }
   inputs_.mask_layer = mask_layer;
   if (inputs_.mask_layer.get()) {
+    // The mask layer should not have any children.
+    DCHECK(inputs_.mask_layer->children().empty());
+
     inputs_.mask_layer->RemoveFromParent();
     DCHECK(!inputs_.mask_layer->parent());
     inputs_.mask_layer->SetParent(this);