V2 app support step2 : Use NativeAppWindow for Activity's window.

athena/content/web_activity.cc

Index: athena/content/web_activity.cc
diff --git a/athena/content/web_activity.cc b/athena/content/web_activity.cc
index 50d71c925d27efe11e4712b55be55b2b74c310f3..6d8b217782d60034a767c513cd72e7b78da87925 100644
--- a/athena/content/web_activity.cc
+++ b/athena/content/web_activity.cc
@@ -472,6 +472,10 @@ views::View* WebActivity::GetContentsView() {
   return web_view_;
 }
 
+views::Widget* WebActivity::CreateWidget() {
+  return NULL;  // Use default widget.
+}
+
 void WebActivity::CreateOverviewModeImage() {
   // TODO(skuhne): Create an overview.
 }