Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/tree_synchronizer_unittest.cc

Index: cc/trees/tree_synchronizer_unittest.cc
diff --git a/cc/trees/tree_synchronizer_unittest.cc b/cc/trees/tree_synchronizer_unittest.cc
index 32add7dcacd836132fc1072bc034b95650b35d98..2c77ef6bab9a7a240d4049ac46be61fd259f135c 100644
--- a/cc/trees/tree_synchronizer_unittest.cc
+++ b/cc/trees/tree_synchronizer_unittest.cc
@@ -226,7 +226,7 @@
   ASSERT_EQ(1u, layer_impl_destruction_list.size());
   EXPECT_EQ(second_layer_impl_id, layer_impl_destruction_list[0]);
 
-  host_->active_tree()->ClearLayers();
+  host_->active_tree()->DetachLayers();
 }
 
 // Constructs a very simple tree and checks that a stacking-order change is
@@ -272,7 +272,7 @@
   EXPECT_FALSE(layer_impl_tree_root->LayerPropertyChanged());
   EXPECT_FALSE(layer_impl_tree_root->children()[0]->LayerPropertyChanged());
   EXPECT_TRUE(layer_impl_tree_root->children()[1]->LayerPropertyChanged());
-  host_->active_tree()->ClearLayers();
+  host_->active_tree()->DetachLayers();
 }
 
 TEST_F(TreeSynchronizerTest, SyncSimpleTreeAndProperties) {
@@ -371,7 +371,7 @@
 
   EXPECT_EQ(0u, layer_impl_destruction_list.size());
 
-  host_->active_tree()->ClearLayers();
+  host_->active_tree()->DetachLayers();
 }
 
 // Constructs a very simple tree, synchronizes it, then synchronizes to a
@@ -486,7 +486,7 @@
   ExpectTreesAreIdentical(layer_tree_root.get(), layer_impl_tree_root,
                           host_->active_tree());
 
-  host_->active_tree()->ClearLayers();
+  host_->active_tree()->DetachLayers();
 }
 
 TEST_F(TreeSynchronizerTest, SynchronizeCurrentlyScrollingNode) {