views: provide feedback when editing fails

ui/views/controls/textfield/textfield.h

Index: ui/views/controls/textfield/textfield.h
diff --git a/ui/views/controls/textfield/textfield.h b/ui/views/controls/textfield/textfield.h
index 22ff37b869acab431c7710407f0566326106d93e..5b21c054eef33157fb979cc6295ba72fe0f72fdc 100644
--- a/ui/views/controls/textfield/textfield.h
+++ b/ui/views/controls/textfield/textfield.h
@@ -392,6 +392,10 @@ class VIEWS_EXPORT Textfield : public View,
   // Pastes the selection clipboard for the specified mouse event.
   void PasteSelectionClipboard(const ui::MouseEvent& event);
 
+  // Called whenever a keypress is unhandled for any reason, including failing
+  // to insert text into a readonly text field.
+  void OnKeypressUnhandled();
+
   // The text model.
   std::unique_ptr<TextfieldModel> model_;