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

sky/shell/ui_delegate.h

Index: sky/shell/ui_delegate.h
diff --git a/sky/shell/ui_delegate.h b/sky/shell/ui_delegate.h
index bb7e44b8c149999c60a8b06e19ac7fcff028496d..d50a34acdf421b329ba7f480f4dbd4114719196a 100644
--- a/sky/shell/ui_delegate.h
+++ b/sky/shell/ui_delegate.h
@@ -5,6 +5,8 @@
 #ifndef SKY_SHELL_UI_DELEGATE_H_
 #define SKY_SHELL_UI_DELEGATE_H_
 
+#include "mojo/public/cpp/bindings/interface_request.h"
+#include "sky/services/viewport/viewport_observer.mojom.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace sky {
@@ -12,8 +14,8 @@ namespace shell {
 
 class UIDelegate {
  public:
-  virtual void OnViewportMetricsChanged(const gfx::Size& size,
-                                        float device_pixel_ratio) = 0;
+  virtual void ConnectToViewportObserver(
+      mojo::InterfaceRequest<ViewportObserver> request) = 0;
 
  protected:
   virtual ~UIDelegate();