Reland 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 7f535e8bf18bcad98a635dfbe14f3195d94a4693..5e50c247594982096c4f9c18c4bf93044614d3a2 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -96,9 +96,8 @@
 }
 
 void LayerTreeImpl::Shutdown() {
-  if (root_layer_)
-    RemoveLayer(root_layer_->id());
-  root_layer_ = nullptr;
+  DetachLayers();
+  DCHECK(!root_layer_);
 }
 
 void LayerTreeImpl::ReleaseResources() {
@@ -315,11 +314,6 @@
   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,