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
UINT message,
                                                      WPARAM wparam,
                                                      LPARAM lparam) {
  switch (message) {
    case WM_CREATE: {
      auto*& cs = reinterpret_cast<const CREATESTRUCT*&>(lparam);
      auto* data =
          static_cast<shared_ptr<NotificationData>*>(cs->lpCreateParams);
      auto* inst = new Toast(hwnd, data);
      SetWindowLongPtr(hwnd, 0, (LONG_PTR)inst);
    } break;
    case WM_NCDESTROY:
      delete Get(hwnd);
      SetWindowLongPtr(hwnd, 0, 0);
      return 0;
    case WM_DESTROY:
      if (Get(hwnd)->uia_) {
        
        UiaReturnRawElementProvider(hwnd, 0, 0, nullptr);
      }
      break;
    case WM_MOUSEACTIVATE:
      return MA_NOACTIVATE;
    case WM_TIMER:
      if (wparam == TimerID_AutoDismiss) {
        Get(hwnd)->AutoDismiss();
      }
      return 0;