cros: Adds ShellPort functions for simplified display mode

ash/host/ash_window_tree_host.h

Index: ash/host/ash_window_tree_host.h
diff --git a/ash/host/ash_window_tree_host.h b/ash/host/ash_window_tree_host.h
index 2459aea36d6a5e46c70af7da6f2bb8d9bb21887c..dfe20f5ec327959aa6a3ddc58896012b04e50f6d 100644
--- a/ash/host/ash_window_tree_host.h
+++ b/ash/host/ash_window_tree_host.h
@@ -29,10 +29,9 @@ class RootWindowTransformer;
 class ASH_EXPORT AshWindowTreeHost {
  public:
   AshWindowTreeHost();
-  virtual ~AshWindowTreeHost() {}
+  virtual ~AshWindowTreeHost();
 
-  // Creates a new AshWindowTreeHost. The caller owns the returned value.
-  static AshWindowTreeHost* Create(
+  static std::unique_ptr<AshWindowTreeHost> Create(
       const AshWindowTreeHostInitParams& init_params);
 
   void set_input_method_handler(InputMethodEventHandler* input_method_handler) {