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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 5e89bbeda7ebed65b7fb2dde7c04b8d84c2f968a..2cd7033b4e6abee45001242843777962fdeaa613 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1183,12 +1183,6 @@ void LayerTreeHostImpl::ResetTreesForTesting() {
   recycle_tree_ = nullptr;
 }
 
-void LayerTreeHostImpl::ResetRecycleTreeForTesting() {
-  if (recycle_tree_)
-    recycle_tree_->DetachLayerTree();
-  recycle_tree_ = nullptr;
-}
-
 void LayerTreeHostImpl::EnforceManagedMemoryPolicy(
     const ManagedMemoryPolicy& policy) {
 
@@ -1286,7 +1280,7 @@ void LayerTreeHostImpl::GetPictureLayerImplPairs(
     if (!layer->HasValidTilePriorities())
       continue;
 
-    PictureLayerImpl* twin_layer = layer->GetTwinLayer();
+    PictureLayerImpl* twin_layer = layer->GetPendingOrActiveTwinLayer();
 
     // Ignore the twin layer when tile priorities are invalid.
     // TODO(vmpstr): Iterators should handle this instead. crbug.com/381704