Refactor SkyShell to allow multiple SkyViews

sky/shell/platform_view.h

Index: sky/shell/platform_view.h
diff --git a/sky/shell/platform_view.h b/sky/shell/platform_view.h
index 59239b90d401c726d87626e2ac456e67b3d9d51f..4fb3618cc59e078cf5cc05ebede5d4a991a4ec57 100644
--- a/sky/shell/platform_view.h
+++ b/sky/shell/platform_view.h
@@ -19,13 +19,16 @@ class PlatformView {
     scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner;
   };
 
-  explicit PlatformView(const Config& config);
+  static PlatformView* Create(const Config& config);
+
   virtual ~PlatformView();
 
   void ConnectToViewportObserver(
       mojo::InterfaceRequest<ViewportObserver> request);
 
  protected:
+  explicit PlatformView(const Config& config);
+
   void SurfaceWasCreated();
   void SurfaceWasDestroyed();