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

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index ea9c1d012f14c9097bbd0dd3b262de1dfe1696bc..6e44bfc80355276bcc249f751725ed695b6fbf89 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -108,9 +108,7 @@
   DCHECK(layers_with_copy_output_request_.empty());
 }
 
-void LayerTreeImpl::Shutdown() {
-  root_layer_ = nullptr;
-}
+void LayerTreeImpl::Shutdown() { root_layer_.reset(); }
 
 void LayerTreeImpl::ReleaseResources() {
   if (root_layer_)
@@ -122,8 +120,8 @@
     inner_viewport_scroll_layer_->SetScrollOffsetDelegate(NULL);
   if (outer_viewport_scroll_layer_)
     outer_viewport_scroll_layer_->SetScrollOffsetDelegate(NULL);
-  inner_viewport_scroll_delegate_proxy_ = nullptr;
-  outer_viewport_scroll_delegate_proxy_ = nullptr;
+  inner_viewport_scroll_delegate_proxy_.reset();
+  outer_viewport_scroll_delegate_proxy_.reset();
 
   root_layer_ = layer.Pass();
   currently_scrolling_layer_ = NULL;
@@ -183,8 +181,8 @@
     inner_viewport_scroll_layer_->SetScrollOffsetDelegate(NULL);
   if (outer_viewport_scroll_layer_)
     outer_viewport_scroll_layer_->SetScrollOffsetDelegate(NULL);
-  inner_viewport_scroll_delegate_proxy_ = nullptr;
-  outer_viewport_scroll_delegate_proxy_ = nullptr;
+  inner_viewport_scroll_delegate_proxy_.reset();
+  outer_viewport_scroll_delegate_proxy_.reset();
   inner_viewport_scroll_layer_ = NULL;
   outer_viewport_scroll_layer_ = NULL;
   page_scale_layer_ = NULL;
@@ -785,17 +783,19 @@
   switch (settings().scrollbar_animator) {
     case LayerTreeSettings::LinearFade: {
       return ScrollbarAnimationControllerLinearFade::Create(
-          scrolling_layer, layer_tree_host_impl_, delay, duration);
+                 scrolling_layer, layer_tree_host_impl_, delay, duration)
+          .PassAs<ScrollbarAnimationController>();
     }
     case LayerTreeSettings::Thinning: {
       return ScrollbarAnimationControllerThinning::Create(
-          scrolling_layer, layer_tree_host_impl_, delay, duration);
+                 scrolling_layer, layer_tree_host_impl_, delay, duration)
+          .PassAs<ScrollbarAnimationController>();
     }
     case LayerTreeSettings::NoAnimator:
       NOTREACHED();
       break;
   }
-  return nullptr;
+  return scoped_ptr<ScrollbarAnimationController>();
 }
 
 void LayerTreeImpl::DidAnimateScrollOffset() {
@@ -881,8 +881,8 @@
       InnerViewportScrollLayer()->SetScrollOffsetDelegate(NULL);
     if (OuterViewportScrollLayer())
       OuterViewportScrollLayer()->SetScrollOffsetDelegate(NULL);
-    inner_viewport_scroll_delegate_proxy_ = nullptr;
-    outer_viewport_scroll_delegate_proxy_ = nullptr;
+    inner_viewport_scroll_delegate_proxy_.reset();
+    outer_viewport_scroll_delegate_proxy_.reset();
   }
 
   root_layer_scroll_offset_delegate_ = root_layer_scroll_offset_delegate;