* Output of `node_modules/.bin/electron --version`: v3.0.6
* Operating System (Platform and Version): Windows 10 (October Update)

**Expected Behavior**
When using the accelerator "Shift+Backspace" in a `Menu`, the accelerator should be displayed as "Shift+Backspace"

**Actual behavior**
"Shift+Shift+Backspace" is displayed as accelerator in the menu.

**To Reproduce**
See: https://github.com/torjusb/electron-accelerator-bug

**Screenshots**
<img width="385" alt="accelerator" src="https://user-images.githubusercontent.com/53583/47494149-49635f80-d851-11e8-8178-eed38f22e0e2.png">

**Additional info**
This only appears to affect "Shift+Backspace" from limited testing. Accelerators such as "Alt+Backspace", "Shift+M" etc. appears to work fine.
atom/browser/api/atom_api_menu.cc
int command_id,
    bool use_default_accelerator,
    ui::Accelerator* accelerator) const {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  v8::Local<v8::Value> val =
      get_accelerator_.Run(GetWrapper(), command_id, use_default_accelerator);
  return mate::ConvertFromV8(isolate(), val, accelerator);
}
void Menu::ExecuteCommand(int command_id, int flags) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  execute_command_.Run(GetWrapper(),
                       mate::internal::CreateEventFromFlags(isolate(), flags),
                       command_id);
}
void Menu::MenuWillShow(ui::SimpleMenuModel* source) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  menu_will_show_.Run(GetWrapper());
}
void Menu::InsertItemAt(int index,
                        int command_id,
                        const base::string16& label) {
  model_->InsertItemAt(index, command_id, label);
}
void Menu::InsertSeparatorAt(int index) {
  model_->InsertSeparatorAt(index, ui::NORMAL_SEPARATOR);
}
void Menu::InsertCheckItemAt(int index,