cc: Always keep the PictureLayerImpl::twin_layer_ pointer valid.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 8f2f0139328ba3465d80fcd6ba0974de2bed86e1..e1f52ef779fbe34e711775e716abf790dcdca736 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -747,13 +747,6 @@ LayerImpl* LayerTreeImpl::FindPendingTreeLayerById(int id) {
   return tree->LayerById(id);
 }
 
-LayerImpl* LayerTreeImpl::FindRecycleTreeLayerById(int id) {
-  LayerTreeImpl* tree = layer_tree_host_impl_->recycle_tree();
-  if (!tree)
-    return NULL;
-  return tree->LayerById(id);
-}
-
 bool LayerTreeImpl::PinchGestureActive() const {
   return layer_tree_host_impl_->pinch_gesture_active();
 }