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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1165 matching lines @@
     active_tree_->DetachLayerTree();
   active_tree_ = LayerTreeImpl::create(this);
   if (pending_tree_)
     pending_tree_->DetachLayerTree();
   pending_tree_ = nullptr;
   if (recycle_tree_)
     recycle_tree_->DetachLayerTree();
   recycle_tree_ = nullptr;
 }
 
-void LayerTreeHostImpl::ResetRecycleTreeForTesting() {
-  if (recycle_tree_)
-    recycle_tree_->DetachLayerTree();
-  recycle_tree_ = nullptr;
-}
-
 void LayerTreeHostImpl::EnforceManagedMemoryPolicy(
     const ManagedMemoryPolicy& policy) {
 
   bool evicted_resources = client_->ReduceContentsTextureMemoryOnImplThread(
       visible_ ? policy.bytes_limit_when_visible : 0,
       ManagedMemoryPolicy::PriorityCutoffToValue(
           visible_ ? policy.priority_cutoff_when_visible
                    : gpu::MemoryAllocation::CUTOFF_ALLOW_NOTHING));
   if (evicted_resources) {
     active_tree_->SetContentsTexturesPurged();
@@ skipping 77 matching lines @@
   for (std::vector<PictureLayerImpl*>::const_iterator it =
            picture_layers_.begin();
        it != picture_layers_.end();
        ++it) {
     PictureLayerImpl* layer = *it;
 
     // TODO(vmpstr): Iterators and should handle this instead. crbug.com/381704
     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
     if (twin_layer && !twin_layer->HasValidTilePriorities())
       twin_layer = NULL;
 
     // If the current tree is ACTIVE_TREE, then always generate a layer_pair.
     // If current tree is PENDING_TREE, then only generate a layer_pair if
     // there is no twin layer.
     if (layer->GetTree() == ACTIVE_TREE) {
@@ skipping 2178 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc