Add GetDefaultIndex() to ComboboxModel.

ui/base/models/combobox_model.cc

Index: ui/base/models/combobox_model.cc
diff --git a/ui/shell_dialogs/select_file_policy.cc b/ui/base/models/combobox_model.cc
similarity index 66%
copy from ui/shell_dialogs/select_file_policy.cc
copy to ui/base/models/combobox_model.cc
index 7e0f5861147dea84903d9ccfd921b80dc304bec4..27b2b477b4d457c829f7e4d6c6764a3db592a072 100644
--- a/ui/shell_dialogs/select_file_policy.cc
+++ b/ui/base/models/combobox_model.cc
@@ -2,10 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "ui/shell_dialogs/select_file_policy.h"
+#include "ui/base/models/combobox_model.h"
 
 namespace ui {
 
-SelectFilePolicy::~SelectFilePolicy() {}
+int ComboboxModel::GetDefaultIndex() const {
+  return 0;
+}
 
 }  // namespace ui