cc: Refactor shared code in tree sync unit tests

cc/tree_synchronizer.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/tree_synchronizer.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/layer.h"
 #include "cc/layer_impl.h"
 #include "cc/scrollbar_animation_controller.h"
 #include "cc/scrollbar_layer.h"
 #include "cc/scrollbar_layer_impl.h"
 
 namespace cc {
 
 scoped_ptr<LayerImpl> TreeSynchronizer::synchronizeTrees(Layer* layerRoot, scoped_ptr<LayerImpl> oldLayerImplRoot, LayerTreeImpl* treeImpl)
 {
+    DCHECK(treeImpl);
+
     TRACE_EVENT0("cc", "TreeSynchronizer::synchronizeTrees");
     ScopedPtrLayerImplMap oldLayers;
     RawPtrLayerImplMap newLayers;
 
     collectExistingLayerImplRecursive(oldLayers, oldLayerImplRoot.Pass());
 
     scoped_ptr<LayerImpl> newTree = synchronizeTreeRecursive(newLayers, oldLayers, layerRoot, treeImpl);
 
     updateScrollbarLayerPointersRecursive(newLayers, layerRoot);
 
@@ skipping 74 matching lines @@
     DCHECK(scrollbarLayerImpl);
     DCHECK(scrollLayerImpl);
 
     if (scrollbarLayerImpl->orientation() == WebKit::WebScrollbar::Horizontal)
         scrollLayerImpl->setHorizontalScrollbarLayer(scrollbarLayerImpl);
     else
         scrollLayerImpl->setVerticalScrollbarLayer(scrollbarLayerImpl);
 }
 
 }  // namespace cc