gn/linux: warnings as errors, turn on Wextra

ash/shelf/shelf_view.h

Index: ash/shelf/shelf_view.h
diff --git a/ash/shelf/shelf_view.h b/ash/shelf/shelf_view.h
index b37cc3e812c224085bc2cd2af1eeaeb5bd2f4a97..4a5868c0a90e00d68332ff1aa8d4ddce0e512cc0 100644
--- a/ash/shelf/shelf_view.h
+++ b/ash/shelf/shelf_view.h
@@ -133,7 +133,7 @@ class ASH_EXPORT ShelfView : public views::View,
   virtual void EndDrag(bool cancel) OVERRIDE;
 
   // Return the view model for test purposes.
-  const views::ViewModel* const view_model_for_test() const {
+  const views::ViewModel* view_model_for_test() const {
     return view_model_.get();
   }