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

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 64758be799b5d6319c6d597a32076180bd23ee42..fb23ba64c651e32c1b4279ee512be0c305915eb3 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -4,6 +4,7 @@
 
 #include "cc/trees/layer_tree_host_impl.h"
 
+#include <algorithm>
 #include <cmath>
 
 #include "base/bind.h"
@@ -7443,47 +7444,64 @@ TEST_F(LayerTreeHostImplTest, ScrollAnimated) {
 
 TEST_F(LayerTreeHostImplTest, GetPictureLayerImplPairs) {
   host_impl_->CreatePendingTree();
+  host_impl_->pending_tree()->SetRootLayer(
+      PictureLayerImpl::Create(host_impl_->pending_tree(), 10));
+
+  LayerTreeImpl* pending_tree = host_impl_->pending_tree();
+  LayerImpl* pending_layer = pending_tree->root_layer();
+
+  std::vector<PictureLayerImpl::Pair> layer_pairs;
+  host_impl_->GetPictureLayerImplPairs(&layer_pairs);
+  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(pending_layer, layer_pairs[0].pending);
+  EXPECT_EQ(nullptr, layer_pairs[0].active);
+
   host_impl_->ActivateSyncTree();
-  host_impl_->CreatePendingTree();
 
   LayerTreeImpl* active_tree = host_impl_->active_tree();
-  LayerTreeImpl* pending_tree = host_impl_->pending_tree();
+  LayerImpl* active_layer = active_tree->root_layer();
   EXPECT_NE(active_tree, pending_tree);
+  EXPECT_NE(active_layer, pending_layer);
+  EXPECT_NE(nullptr, active_tree);
+  EXPECT_NE(nullptr, active_layer);
 
-  scoped_ptr<FakePictureLayerImpl> active_layer =
-      FakePictureLayerImpl::Create(active_tree, 10);
-  scoped_ptr<FakePictureLayerImpl> pending_layer =
-      FakePictureLayerImpl::Create(pending_tree, 10);
+  host_impl_->CreatePendingTree();
 
-  std::vector<PictureLayerImpl::Pair> layer_pairs;
+  layer_pairs.clear();
   host_impl_->GetPictureLayerImplPairs(&layer_pairs);
+  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(active_layer, layer_pairs[0].active);
+  EXPECT_EQ(pending_layer, layer_pairs[0].pending);
 
-  EXPECT_EQ(2u, layer_pairs.size());
-  if (layer_pairs[0].active) {
-    EXPECT_EQ(active_layer.get(), layer_pairs[0].active);
-    EXPECT_EQ(NULL, layer_pairs[0].pending);
-  } else {
-    EXPECT_EQ(pending_layer.get(), layer_pairs[0].pending);
-    EXPECT_EQ(NULL, layer_pairs[0].active);
-  }
+  // Activate, the active layer has no twin now.
+  host_impl_->ActivateSyncTree();
 
-  if (layer_pairs[1].active) {
-    EXPECT_EQ(active_layer.get(), layer_pairs[1].active);
-    EXPECT_EQ(NULL, layer_pairs[1].pending);
-  } else {
-    EXPECT_EQ(pending_layer.get(), layer_pairs[1].pending);
-    EXPECT_EQ(NULL, layer_pairs[1].active);
-  }
+  layer_pairs.clear();
+  host_impl_->GetPictureLayerImplPairs(&layer_pairs);
+  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(active_layer, layer_pairs[0].active);
+  EXPECT_EQ(nullptr, layer_pairs[0].pending);
+
+  // Create another layer in the pending tree that's not in the active tree. We
+  // should get two pairs.
+  host_impl_->CreatePendingTree();
+  host_impl_->pending_tree()->root_layer()->AddChild(
+      PictureLayerImpl::Create(host_impl_->pending_tree(), 11));
 
-  active_layer->set_twin_layer(pending_layer.get());
-  pending_layer->set_twin_layer(active_layer.get());
+  LayerImpl* new_pending_layer = pending_tree->root_layer()->children()[0];
 
   layer_pairs.clear();
   host_impl_->GetPictureLayerImplPairs(&layer_pairs);
-  EXPECT_EQ(1u, layer_pairs.size());
+  EXPECT_EQ(2u, layer_pairs.size());
+
+  // The pair ordering is flaky, so make it consistent.
+  if (layer_pairs[0].active != active_layer)
+    std::swap(layer_pairs[0], layer_pairs[1]);
 
-  EXPECT_EQ(active_layer.get(), layer_pairs[0].active);
-  EXPECT_EQ(pending_layer.get(), layer_pairs[0].pending);
+  EXPECT_EQ(active_layer, layer_pairs[0].active);
+  EXPECT_EQ(pending_layer, layer_pairs[0].pending);
+  EXPECT_EQ(new_pending_layer, layer_pairs[1].pending);
+  EXPECT_EQ(nullptr, layer_pairs[1].active);
 }
 
 TEST_F(LayerTreeHostImplTest, DidBecomeActive) {