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

athena/content/app_activity_proxy.cc

Index: athena/content/app_activity_proxy.cc
diff --git a/athena/content/app_activity_proxy.cc b/athena/content/app_activity_proxy.cc
index bf08a592c1cb28d141630ccd6229e95978b2e1f7..491aaefd3809e357839e41eff72a96a3375a8d34 100644
--- a/athena/content/app_activity_proxy.cc
+++ b/athena/content/app_activity_proxy.cc
@@ -84,6 +84,10 @@ views::View* AppActivityProxy::GetContentsView() {
   return view_;
 }
 
+views::Widget* AppActivityProxy::CreateWidget() {
+  return NULL;
+}
+
 void AppActivityProxy::CreateOverviewModeImage() {
   // Nothing we can do here.
 }