Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/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/trees/tree_synchronizer.h"
 
 #include <stddef.h>
 
 #include <set>
 
@@ skipping 25 matching lines @@
       // Need to ensure that layer destruction doesn't tear down other layers
       // linked to this LayerImpl that have been used in the new tree.
       it.second->ClearLinksToOtherLayers();
     }
   }
 }
 
 void TreeSynchronizer::SynchronizeTrees(Layer* layer_root,
                                         LayerTreeImpl* tree_impl) {
   if (!layer_root)
-    tree_impl->ClearLayers();
+    tree_impl->DetachLayers();
   else
     SynchronizeTreesInternal(layer_root, tree_impl);
 }
 
 void TreeSynchronizer::SynchronizeTrees(LayerImpl* layer_root,
                                         LayerTreeImpl* tree_impl) {
   if (!layer_root)
-    tree_impl->ClearLayers();
+    tree_impl->DetachLayers();
   else
     SynchronizeTreesInternal(layer_root, tree_impl);
 }
 
 template <typename LayerType>
 std::unique_ptr<LayerImpl> ReuseOrCreateLayerImpl(OwnedLayerImplMap* old_layers,
                                                   LayerType* layer,
                                                   LayerTreeImpl* tree_impl) {
   if (!layer)
     return nullptr;
@@ skipping 76 matching lines @@
                               active_tree);
 }
 
 void TreeSynchronizer::PushLayerProperties(LayerTreeHost* host_tree,
                                            LayerTreeImpl* impl_tree) {
   PushLayerPropertiesInternal(host_tree->LayersThatShouldPushProperties(),
                               impl_tree);
 }
 
 }  // namespace cc