Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/layer_tree_impl.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/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 78 matching lines @@
 LayerTreeImpl::~LayerTreeImpl() {
   BreakSwapPromises(IsActiveTree() ? SwapPromise::SWAP_FAILS
                                    : SwapPromise::ACTIVATION_FAILS);
 
   // Need to explicitly clear the tree prior to destroying this so that
   // the LayerTreeImpl pointer is still valid in the LayerImpl dtor.
   DCHECK(!root_layer_);
 }
 
 void LayerTreeImpl::Shutdown() {
-  if (root_layer_)
-    RemoveLayer(root_layer_->id());
-  root_layer_ = nullptr;
+  DetachLayers();
+  DCHECK(!root_layer_);
 }
 
 void LayerTreeImpl::ReleaseResources() {
   if (root_layer_) {
     LayerTreeHostCommon::CallFunctionForEveryLayer(
         this, [](LayerImpl* layer) { layer->ReleaseResources(); });
   }
 }
 
 void LayerTreeImpl::RecreateResources() {
@@ skipping 198 matching lines @@
 
 std::unique_ptr<OwnedLayerImplList> LayerTreeImpl::DetachLayers() {
   root_layer_ = nullptr;
   render_surface_layer_list_.clear();
   set_needs_update_draw_properties();
   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,
                                                 ClipTree* clip_tree) {
   if (layer && layer->masks_to_bounds()) {
     ClipNode* clip_node = clip_tree->Node(layer->clip_tree_index());
     if (clip_node) {
       DCHECK_EQ(layer->id(), clip_node->owner_id);
       gfx::SizeF bounds = gfx::SizeF(layer->bounds());
       if (clip_node->data.clip.size() != bounds) {
         clip_node->data.clip.set_size(bounds);
         clip_tree->set_needs_update(true);
@@ skipping 1734 matching lines @@
 }
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   for (auto* layer : *this)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc