* 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
callback_ = callback;
    g_signal_connect(dialog_, "delete-event",
                     G_CALLBACK(gtk_widget_hide_on_delete), nullptr);
    g_signal_connect(dialog_, "response", G_CALLBACK(OnResponseDialogThunk),
                     this);
    Show();
  }
  void OnWindowClosed() override {
    parent_->RemoveObserver(this);
    parent_ = nullptr;
  }
  CHROMEGTK_CALLBACK_1(GtkMessageBox, void, OnResponseDialog, int);
  CHROMEGTK_CALLBACK_0(GtkMessageBox, void, OnCheckboxToggled);
 private:
  atom::UnresponsiveSuppressor unresponsive_suppressor_;
  
  int cancel_id_ = 0;
  bool checkbox_checked_ = false;
  NativeWindow* parent_;
  GtkWidget* dialog_;
  MessageBoxCallback callback_;
  DISALLOW_COPY_AND_ASSIGN(GtkMessageBox);
};
void GtkMessageBox::OnResponseDialog(GtkWidget* widget, int response) {