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
dwpFlags);
  return hdwp;
}
void DesktopNotificationController::Toast::StartEaseIn() {
  DCHECK(!ease_in_active_);
  ease_in_start_ = GetTickCount();
  ease_in_active_ = true;
  data_->controller->StartAnimation();
}
void DesktopNotificationController::Toast::StartEaseOut() {
  DCHECK(!ease_out_active_);
  ease_out_start_ = GetTickCount();
  ease_out_active_ = true;
  data_->controller->StartAnimation();
}
bool DesktopNotificationController::Toast::IsStackCollapseActive() const {
  return (vertical_pos_ != vertical_pos_target_);
}
float DesktopNotificationController::Toast::AnimateEaseIn() {
  if (!ease_in_active_)
    return ease_in_pos_;
  constexpr DWORD duration = 500;
  auto elapsed = GetTickCount() - ease_in_start_;
  float time = std::min(duration, elapsed) / static_cast<float>(duration);
  
  const float a = -8.0f;
  auto pos = (std::exp(a * time) - 1.0f) / (std::exp(a) - 1.0f);
  return pos;
}
float DesktopNotificationController::Toast::AnimateEaseOut() {