Revert of cc: Remove use of PassAs() and constructor-casting with scoped_ptr.

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 224513c2a59f0ee51476eeed69a91a31249b4737..29bdfe778fdf522fc9a3a8f97d74ef4adb90b71c 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -119,7 +119,7 @@
       return ret.Pass();
     }
   }
-  return nullptr;
+  return scoped_ptr<LayerImpl>();
 }
 
 void LayerImpl::SetParent(LayerImpl* parent) {
@@ -246,7 +246,7 @@
 }
 
 void LayerImpl::ClearRenderSurface() {
-  draw_properties_.render_surface = nullptr;
+  draw_properties_.render_surface.reset();
 }
 
 void LayerImpl::ClearRenderSurfaceLayerList() {
@@ -1324,7 +1324,7 @@
 
   bool need_scrollbar_animation_controller = scrollable() && scrollbars_;
   if (!need_scrollbar_animation_controller) {
-    scrollbar_animation_controller_ = nullptr;
+    scrollbar_animation_controller_.reset();
     return;
   }
 
@@ -1358,7 +1358,7 @@
 
   scrollbars_->erase(layer);
   if (scrollbars_->empty())
-    scrollbars_ = nullptr;
+    scrollbars_.reset();
 }
 
 bool LayerImpl::HasScrollbar(ScrollbarOrientation orientation) const {