* 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
#include "atom/browser/ui/message_box.h"
#include <glib/gi18n.h>
#include "atom/browser/browser.h"
#include "atom/browser/native_window_observer.h"
#include "atom/browser/native_window_views.h"
#include "atom/browser/unresponsive_suppressor.h"
#include "base/callback.h"
#include "base/strings/string_util.h"
#include "base/strings/utf_string_conversions.h"
#include "chrome/browser/ui/libgtkui/gtk_signal.h"
#include "chrome/browser/ui/libgtkui/gtk_util.h"
#include "chrome/browser/ui/libgtkui/skia_utils_gtk.h"
#include "ui/gfx/image/image_skia.h"
#include "ui/views/widget/desktop_aura/x11_desktop_handler.h"
#define ANSI_FOREGROUND_RED "\x1b[31m"
#define ANSI_FOREGROUND_BLACK "\x1b[30m"
#define ANSI_TEXT_BOLD "\x1b[1m"
#define ANSI_BACKGROUND_GRAY "\x1b[47m"
#define ANSI_RESET "\x1b[0m"
namespace atom {
namespace {
class GtkMessageBox : public NativeWindowObserver {