Fix the Views combobox on GTK by adding a new function.

views/controls/combobox/native_combobox_gtk.cc

Index: views/controls/combobox/native_combobox_gtk.cc
diff --git a/views/controls/combobox/native_combobox_gtk.cc b/views/controls/combobox/native_combobox_gtk.cc
index 3a3b6f12a33ed6d0c912d32559debbd11a9ffe9c..e1a291751fb849d7fc128b307cb85f148ba01590 100644
--- a/views/controls/combobox/native_combobox_gtk.cc
+++ b/views/controls/combobox/native_combobox_gtk.cc
@@ -57,6 +57,10 @@ void NativeComboboxGtk::SetFocus() {
   Focus();
 }
 
+gfx::NativeView NativeComboboxGtk::GetTestingHandle() const {
+  return native_view();
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // NativeComboboxGtk, NativeControlGtk overrides: