Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 479021eb0544c09d4a6efdf4899d2b408181394f..35eb9531aca33e14fab4d1c08152d085c712784a 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1215,18 +1215,18 @@
 
 void LayerTreeHostImpl::ResetTreesForTesting() {
   if (active_tree_)
-    active_tree_->ClearLayers();
+    active_tree_->DetachLayers();
   active_tree_ =
       LayerTreeImpl::create(this, active_tree()->page_scale_factor(),
                             active_tree()->top_controls_shown_ratio(),
                             active_tree()->elastic_overscroll());
   active_tree_->property_trees()->is_active = true;
   if (pending_tree_)
-    pending_tree_->ClearLayers();
+    pending_tree_->DetachLayers();
   pending_tree_ = nullptr;
   pending_tree_duration_timer_ = nullptr;
   if (recycle_tree_)
-    recycle_tree_->ClearLayers();
+    recycle_tree_->DetachLayers();
   recycle_tree_ = nullptr;
 }