* 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
public:
  GtkMessageBox(NativeWindow* parent_window,
                MessageBoxType type,
                const std::vector<std::string>& buttons,
                int default_id,
                int cancel_id,
                const std::string& title,
                const std::string& message,
                const std::string& detail,
                const std::string& checkbox_label,
                bool checkbox_checked)
      : cancel_id_(cancel_id),
        parent_(static_cast<NativeWindow*>(parent_window)) {
    
    dialog_ =
        gtk_message_dialog_new(nullptr,                         
                               static_cast<GtkDialogFlags>(0),  
                               GetMessageType(type),            
                               GTK_BUTTONS_NONE,                
                               "%s", message.c_str());
    if (!detail.empty())
      gtk_message_dialog_format_secondary_text(GTK_MESSAGE_DIALOG(dialog_),