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

cc/trees/layer_tree_host_unittest_picture.cc

 // Copyright 2013 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.h"
 
 #include "cc/test/fake_content_layer_client.h"
 #include "cc/test/fake_picture_layer.h"
 #include "cc/test/fake_picture_layer_impl.h"
 #include "cc/test/layer_tree_test.h"
@@ skipping 48 matching lines @@
     if (pending_root_impl->children().empty()) {
       EXPECT_EQ(2, activates_);
       return;
     }
 
     FakePictureLayerImpl* pending_picture_impl =
         static_cast<FakePictureLayerImpl*>(pending_root_impl->children()[0]);
 
     if (!active_root_impl) {
       EXPECT_EQ(0, activates_);
-      EXPECT_EQ(NULL, pending_picture_impl->twin_layer());
+      EXPECT_EQ(nullptr, pending_picture_impl->GetPendingOrActiveTwinLayer());
       return;
     }
 
     if (active_root_impl->children().empty()) {
       EXPECT_EQ(3, activates_);
-      EXPECT_EQ(NULL, pending_picture_impl->twin_layer());
+      EXPECT_EQ(nullptr, pending_picture_impl->GetPendingOrActiveTwinLayer());
       return;
     }
 
     FakePictureLayerImpl* active_picture_impl =
         static_cast<FakePictureLayerImpl*>(active_root_impl->children()[0]);
 
     // After the first activation, when we commit again, we'll have a pending
     // and active layer. Then we recreate a picture layer in the 4th activate
     // and the next commit will have a pending and active twin again.
     EXPECT_TRUE(activates_ == 1 || activates_ == 4);
 
-    EXPECT_EQ(pending_picture_impl, active_picture_impl->twin_layer());
-    EXPECT_EQ(active_picture_impl, pending_picture_impl->twin_layer());
+    EXPECT_EQ(pending_picture_impl,
+              active_picture_impl->GetPendingOrActiveTwinLayer());
+    EXPECT_EQ(active_picture_impl,
+              pending_picture_impl->GetPendingOrActiveTwinLayer());
+    EXPECT_EQ(nullptr, active_picture_impl->GetRecycledTwinLayer());
   }
 
   void DidActivateTreeOnThread(LayerTreeHostImpl* impl) override {
     LayerImpl* active_root_impl = impl->active_tree()->root_layer();
+    LayerImpl* recycle_root_impl = impl->recycle_tree()->root_layer();
 
     if (active_root_impl->children().empty()) {
       EXPECT_EQ(2, activates_);
     } else {
       FakePictureLayerImpl* active_picture_impl =
           static_cast<FakePictureLayerImpl*>(active_root_impl->children()[0]);
+      FakePictureLayerImpl* recycle_picture_impl =
+          static_cast<FakePictureLayerImpl*>(recycle_root_impl->children()[0]);
 
-      EXPECT_EQ(NULL, active_picture_impl->twin_layer());
+      EXPECT_EQ(nullptr, active_picture_impl->GetPendingOrActiveTwinLayer());
+      EXPECT_EQ(recycle_picture_impl,
+                active_picture_impl->GetRecycledTwinLayer());
     }
 
     ++activates_;
     if (activates_ <= 4)
       PostSetNeedsCommitToMainThread();
     else
       EndTest();
   }
 
   void AfterTest() override {}
 
   FakeContentLayerClient client_;
   int activates_;
 };
 
 MULTI_THREAD_TEST_F(LayerTreeHostPictureTestTwinLayer);
 
 }  // namespace
 }  // namespace cc