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

athena/activity/public/activity_view_model.h

Index: athena/activity/public/activity_view_model.h
diff --git a/athena/activity/public/activity_view_model.h b/athena/activity/public/activity_view_model.h
index 12b8ceda8e17324cca7489e6742273d0988cec22..34062447bd6d448ff6eed16d036960f7d45301e5 100644
--- a/athena/activity/public/activity_view_model.h
+++ b/athena/activity/public/activity_view_model.h
@@ -16,6 +16,7 @@ class ImageSkia;
 
 namespace views {
 class View;
+class Widget;
 }
 
 namespace athena {
@@ -46,6 +47,10 @@ class ATHENA_EXPORT ActivityViewModel {
   // be deleted by the resource manager.
   virtual views::View* GetContentsView() = 0;
 
+  // Creates a custom widget for the activity. Returns NULL to use default
+  // implementation.
+  virtual views::Widget* CreateWidget() = 0;
+
   // This gets called before the Activity gets (partially) thrown out of memory
   // to create a preview image of the activity. Note that even if this function
   // gets called, |GetOverviewModeImage()| could still return an empty image.