The browser-process notifications are either missing or reusing images from the last notification. The reason is simple: In [atom_api_notification, we set the](base::TimeTicks::Now()) `icon_url` to `GURL()`, but in the notification presenter, [we assume an okay and unique path](https://github.com/electron/electron/blob/master/brightray/browser/win/notification_presenter_win.cc#L68-L73).

* Electron version: 1.7+
* Operating system: Windows

### Expected behavior
Notifications should contain the passed icon

### Actual behavior
Notifications contain either no or the last passed icon

### How to reproduce
Use browser-process notifications and attach an image on Windows 10 as icon. Now, send another notification with a different image. Observe how the first icon is shown.

brightray/browser/win/notification_presenter_win.cc
}
NotificationPresenterWin::~NotificationPresenterWin() {
}
bool NotificationPresenterWin::Init() {
  return temp_dir_.CreateUniqueTempDir();
}
base::string16 NotificationPresenterWin::SaveIconToFilesystem(
    const SkBitmap& icon, const GURL& origin) {
  std::string filename = base::MD5String(origin.spec()) + ".png";
  base::FilePath path = temp_dir_.GetPath().Append(base::UTF8ToUTF16(filename));
  if (base::PathExists(path))
    return path.value();
  if (SaveIconToPath(icon, path))
    return path.value();
  return base::UTF8ToUTF16(origin.spec());
}
Notification* NotificationPresenterWin::CreateNotificationObject(
    NotificationDelegate* delegate) {
  return new WindowsToastNotification(delegate, this);
}
}