Revert of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 5e50c247594982096c4f9c18c4bf93044614d3a2..7f535e8bf18bcad98a635dfbe14f3195d94a4693 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -96,8 +96,9 @@
 }
 
 void LayerTreeImpl::Shutdown() {
-  DetachLayers();
-  DCHECK(!root_layer_);
+  if (root_layer_)
+    RemoveLayer(root_layer_->id());
+  root_layer_ = nullptr;
 }
 
 void LayerTreeImpl::ReleaseResources() {
@@ -314,6 +315,11 @@
   std::unique_ptr<OwnedLayerImplList> ret = std::move(layers_);
   layers_.reset(new OwnedLayerImplList);
   return ret;
+}
+
+void LayerTreeImpl::ClearLayers() {
+  SetRootLayer(nullptr);
+  DCHECK(layers_->empty());
 }
 
 static void UpdateClipTreeForBoundsDeltaOnLayer(LayerImpl* layer,