Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/layers/picture_layer_impl_unittest.cc

 // Copyright 2013 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/layers/picture_layer_impl.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <limits>
@@ skipping 4190 matching lines @@
   EXPECT_TRUE(pending_layer()->GetPendingOrActiveTwinLayer());
   EXPECT_TRUE(active_layer()->GetPendingOrActiveTwinLayer());
   EXPECT_EQ(pending_layer(), active_layer()->GetPendingOrActiveTwinLayer());
   EXPECT_EQ(active_layer(), pending_layer()->GetPendingOrActiveTwinLayer());
 
   ActivateTree();
   EXPECT_FALSE(active_layer()->GetPendingOrActiveTwinLayer());
 
   // Make an empty pending tree.
   host_impl()->CreatePendingTree();
-  host_impl()->pending_tree()->ClearLayers();
+  host_impl()->pending_tree()->DetachLayers();
   EXPECT_FALSE(active_layer()->GetPendingOrActiveTwinLayer());
 }
 
 void PictureLayerImplTest::TestQuadsForSolidColor(bool test_for_solid) {
   host_impl()->AdvanceToNextFrame(base::TimeDelta::FromMilliseconds(1));
 
   gfx::Size tile_size(100, 100);
   gfx::Size layer_bounds(200, 200);
   gfx::Rect layer_rect(layer_bounds);
 
@@ skipping 540 matching lines @@
   // New low res tiling.
   EXPECT_TRUE(tilings->tiling_at(2)->may_contain_low_resolution_tiles());
 
   // This tiling will be high res now, it won't contain low res content since it
   // was all destroyed.
   EXPECT_FALSE(tilings->tiling_at(1)->may_contain_low_resolution_tiles());
 }
 
 }  // namespace
 }  // namespace cc