Port touch-demo.sky to Dart and make it work in SkyShell

sky/shell/platform_view.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_SHELL_SKY_VIEW_H_
 #define SKY_SHELL_SKY_VIEW_H_
 
 #include "base/android/jni_weak_ref.h"
 #include "base/android/scoped_java_ref.h"
 #include "base/macros.h"
@@ skipping 15 matching lines @@
 
     base::WeakPtr<UIDelegate> ui_delegate;
     scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner;
   };
 
   static bool Register(JNIEnv* env);
 
   explicit PlatformView(const Config& config);
   ~PlatformView();
 
+  void ConnectToViewportObserver(
+      mojo::InterfaceRequest<ViewportObserver> request);
+
   // Called from Java
   void Detach(JNIEnv* env, jobject obj);
   void SurfaceCreated(JNIEnv* env, jobject obj, jobject jsurface);
   void SurfaceDestroyed(JNIEnv* env, jobject obj);
   void SurfaceSetSize(JNIEnv* env,
                       jobject obj,
                       jint width,
                       jint height,
                       jfloat density);
 
  private:
   void ReleaseWindow();
 
   Config config_;
   ANativeWindow* window_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformView);
 };
 
 }  // namespace shell
 }  // namespace sky
 
 #endif  // SKY_SHELL_SKY_VIEW_H_