After creating a notification and passing a tag, then subsequent notifications with the same tag don't show up for the entire lifespan of the app.

* Electron version: 1.8.1
* Operating system: Windows 7 x64

### Expected behavior

New notifications should open after the last one has closed.

### Actual behavior

Nothing happens

### How to reproduce

  $ git clone https://github.com/electron/electron-quick-start .
  $ npm install
  $ npm start

Then in the app open dev tools and paste this in the console:

    new Notification("test", { tag: "test" });

Wait for the notification to disappear and paste it again, observe nothingness.
atom/browser/notifications/win/win32_desktop_notifications/toast.cc
src += 4;
      }
      
      result_bitmap =
          CreateDIBitmap(hdc_screen, &bmi, CBM_INIT, color_bits,
                         reinterpret_cast<BITMAPINFO*>(&bmi), DIB_RGB_COLORS);
    }
    if (hdc_src)
      DeleteDC(hdc_src);
    if (hdc)
      DeleteDC(hdc);
    if (alpha_bitmap)
      DeleteObject(alpha_bitmap);
    if (color_bitmap)
      DeleteObject(color_bitmap);
    DeleteObject(alpha_src_bitmap);
  }
  ReleaseDC(NULL, hdc_screen);
  return result_bitmap;
}
const TCHAR DesktopNotificationController::Toast::class_name_[] =
    TEXT("DesktopNotificationToast");
DesktopNotificationController::Toast::Toast(HWND hwnd,
                                            shared_ptr<NotificationData>* data)
    : hwnd_(hwnd), data_(*data) {
  HDC hdc_screen = GetDC(NULL);
  hdc_ = CreateCompatibleDC(hdc_screen);
  ReleaseDC(NULL, hdc_screen);
}
DesktopNotificationController::Toast::~Toast() {