Revert of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/tree_synchronizer.cc

Index: cc/trees/tree_synchronizer.cc
diff --git a/cc/trees/tree_synchronizer.cc b/cc/trees/tree_synchronizer.cc
index ef403834b93ebd97da690dac46328f346ec45da1..c726f83f8a9f2647de16a05a7aab57d8c0a17aed 100644
--- a/cc/trees/tree_synchronizer.cc
+++ b/cc/trees/tree_synchronizer.cc
@@ -43,7 +43,7 @@
 void TreeSynchronizer::SynchronizeTrees(Layer* layer_root,
                                         LayerTreeImpl* tree_impl) {
   if (!layer_root)
-    tree_impl->DetachLayers();
+    tree_impl->ClearLayers();
   else
     SynchronizeTreesInternal(layer_root, tree_impl);
 }
@@ -51,7 +51,7 @@
 void TreeSynchronizer::SynchronizeTrees(LayerImpl* layer_root,
                                         LayerTreeImpl* tree_impl) {
   if (!layer_root)
-    tree_impl->DetachLayers();
+    tree_impl->ClearLayers();
   else
     SynchronizeTreesInternal(layer_root, tree_impl);
 }