* Output of `node_modules/.bin/electron --version`: v2.0.5
* Operating System (Platform and Version): Ubuntu 18.04LTS

### Expected behavior

alert box should show the app Icon.

### Actual behavior

alert box is showing no icon even though an app icon is set.
![screenshot from 2018-10-22 18-46-46](https://user-images.githubusercontent.com/15051871/47294523-f5773180-d62a-11e8-9a47-6a62bebd4a1d.png)


### How to reproduce
* Open any electron app in debug mode (or run the app directly from the source).
* in the developer console, type `alert('some_string')`
* A dialog box appears with 'some_string' in it and a placeholder for the icon instead of the icon itself.
Refer the image above.

atom/browser/ui/message_box_gtk.cc
"%s", detail.c_str());
    if (!title.empty())
      gtk_window_set_title(GTK_WINDOW(dialog_), title.c_str());
    if (!checkbox_label.empty()) {
      GtkWidget* message_area =
          gtk_message_dialog_get_message_area(GTK_MESSAGE_DIALOG(dialog_));
      GtkWidget* check_button =
          gtk_check_button_new_with_label(checkbox_label.c_str());
      g_signal_connect(check_button, "toggled",
                       G_CALLBACK(OnCheckboxToggledThunk), this);
      gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(check_button),
                                   checkbox_checked);
      gtk_container_add(GTK_CONTAINER(message_area), check_button);
    }
    
    for (size_t i = 0; i < buttons.size(); ++i) {
      GtkWidget* button = gtk_dialog_add_button(
          GTK_DIALOG(dialog_), TranslateToStock(i, buttons[i]), i);
      if (static_cast<int>(i) == default_id)
        gtk_widget_grab_focus(button);
    }
    
    if (parent_) {
      parent_->AddObserver(this);
      static_cast<NativeWindowViews*>(parent_)->SetEnabled(false);
      libgtkui::SetGtkTransientForAura(dialog_, parent_->GetNativeWindow());
      gtk_window_set_modal(GTK_WINDOW(dialog_), TRUE);
    }
  }
  ~GtkMessageBox() override {