Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/damage_tracker_unittest.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/damage_tracker.h"
 
 #include <stddef.h>
 
 #include "cc/base/math_util.h"
 #include "cc/layers/layer_impl.h"
@@ skipping 65 matching lines @@
 }
 
 class DamageTrackerTest : public testing::Test {
  public:
   DamageTrackerTest()
       : host_impl_(&task_runner_provider_,
                    &shared_bitmap_manager_,
                    &task_graph_runner_) {}
 
   LayerImpl* CreateTestTreeWithOneSurface() {
-    host_impl_.active_tree()->ClearLayers();
+    host_impl_.active_tree()->DetachLayers();
     std::unique_ptr<LayerImpl> root =
         LayerImpl::Create(host_impl_.active_tree(), 1);
     std::unique_ptr<LayerImpl> child =
         LayerImpl::Create(host_impl_.active_tree(), 2);
 
     root->SetPosition(gfx::PointF());
     root->SetBounds(gfx::Size(500, 500));
     root->SetDrawsContent(true);
     root->test_properties()->force_render_surface = true;
 
     child->SetPosition(gfx::PointF(100.f, 100.f));
     child->SetBounds(gfx::Size(30, 30));
     child->SetDrawsContent(true);
     root->AddChild(std::move(child));
     host_impl_.active_tree()->SetRootLayer(std::move(root));
 
     return host_impl_.active_tree()->root_layer();
   }
 
   LayerImpl* CreateTestTreeWithTwoSurfaces() {
     // This test tree has two render surfaces: one for the root, and one for
     // child1. Additionally, the root has a second child layer, and child1 has
     // two children of its own.
 
-    host_impl_.active_tree()->ClearLayers();
+    host_impl_.active_tree()->DetachLayers();
     std::unique_ptr<LayerImpl> root =
         LayerImpl::Create(host_impl_.active_tree(), 1);
     std::unique_ptr<LayerImpl> child1 =
         LayerImpl::Create(host_impl_.active_tree(), 2);
     std::unique_ptr<LayerImpl> child2 =
         LayerImpl::Create(host_impl_.active_tree(), 3);
     std::unique_ptr<LayerImpl> grand_child1 =
         LayerImpl::Create(host_impl_.active_tree(), 4);
     std::unique_ptr<LayerImpl> grand_child2 =
         LayerImpl::Create(host_impl_.active_tree(), 5);
@@ skipping 1503 matching lines @@
     gfx::Rect root_damage_rect =
         root->render_surface()->damage_tracker()->current_damage_rect();
     gfx::Rect damage_we_care_about = gfx::Rect(i, i);
     EXPECT_LE(damage_we_care_about.right(), root_damage_rect.right());
     EXPECT_LE(damage_we_care_about.bottom(), root_damage_rect.bottom());
   }
 }
 
 }  // namespace
 }  // namespace cc