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

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 1326b797c940cd60b54e2ba0d51fe21e9f4b0e03..1a22d9254a9b5bded9b1560c4769760efa57779d 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -620,7 +620,7 @@
 void Layer::RemoveScrollChild(Layer* child) {
   scroll_children_->erase(child);
   if (scroll_children_->empty())
-    scroll_children_ = nullptr;
+    scroll_children_.reset();
   SetNeedsCommit();
 }
 
@@ -650,7 +650,7 @@
 void Layer::RemoveClipChild(Layer* child) {
   clip_children_->erase(child);
   if (clip_children_->empty())
-    clip_children_ = nullptr;
+    clip_children_.reset();
   SetNeedsCommit();
 }
 
@@ -1085,7 +1085,7 @@
 }
 
 void Layer::ClearRenderSurface() {
-  draw_properties_.render_surface = nullptr;
+  draw_properties_.render_surface.reset();
 }
 
 void Layer::ClearRenderSurfaceLayerList() {