Reland of cc : Make LayerImpl destruction independent of tree hierarchy

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 6debeb58f2857665473642649c2a8f0488ab4b39..d61b7c76ceffa732f40a84f6f1c3502042423a9b 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -538,6 +538,7 @@
 
   // Toggle the root layer to make sure it toggles can_draw
   host_impl_->active_tree()->SetRootLayer(nullptr);
+  host_impl_->active_tree()->DetachLayers();
   EXPECT_FALSE(host_impl_->CanDraw());
   EXPECT_TRUE(on_can_draw_state_changed_called_);
   on_can_draw_state_changed_called_ = false;
@@ -741,7 +742,7 @@
             host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
                                     InputHandler::WHEEL)
                 .thread);
-  host_impl_->active_tree()->ClearLayers();
+  host_impl_->active_tree()->DetachLayers();
 
   scroll_layer = SetupScrollAndContentsLayers(gfx::Size(100, 100));
 
@@ -5344,7 +5345,7 @@
   // synchronization.
   SetNeedsRebuildPropertyTrees();
   DrawFrame();
-  host_impl_->active_tree()->ClearLayers();
+  host_impl_->active_tree()->DetachLayers();
   std::unique_ptr<LayerImpl> root_ptr2 =
       LayerImpl::Create(host_impl_->active_tree(), 4);
   std::unique_ptr<LayerImpl> root_clip2 =
@@ -7355,8 +7356,7 @@
   EXPECT_GT(context3d->NumTextures(), 0u);
 
   // Kill the layer tree.
-  host_impl_->active_tree()->SetRootLayer(
-      LayerImpl::Create(host_impl_->active_tree(), 100));
+  host_impl_->active_tree()->DetachLayers();
   // There should be no textures left in use after.
   EXPECT_EQ(0u, context3d->NumTextures());
 }