Make it possible to navigate from a SkyView to a WebView.

sky/shell/ui/engine.cc

Index: sky/shell/ui/engine.cc
diff --git a/sky/shell/ui/engine.cc b/sky/shell/ui/engine.cc
index 736cb3e07c37a985badaa1fffbc786e7bc4e94f1..f82922b8fcdcdaa1c4c65982a34df8d65e45b5cc 100644
--- a/sky/shell/ui/engine.cc
+++ b/sky/shell/ui/engine.cc
@@ -185,6 +185,9 @@ void Engine::LoadURL(const mojo::String& mojo_url) {
     return;
   }
 
+  if (sky_view_)
+    sky_view_ = nullptr;
+
   LOG(WARNING) << ".sky support is deprecated, please use .dart for main()";
 
   // Something bad happens if you try to call WebView::close and replace