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

This would occur when bringing SkyDemo to the front after
having viewed a SkyView based demo and then paused
the SkyActivity (by going to the Android Launcher, etc.)

I also fixed deploy_domokit_site.py to ignore .gitignore files
found in packages.  We should actually probably ignore these
when creating the packages in the first place, but
this check seems valid regardless.  The .gitignore for
sky/lib/assets was causing the assets not to get pushed
to the demo site.

R=abarth@chromium.org, johnmccutchan@google.com

Review URL: https://codereview.chromium.org/1163323007.

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