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
ulw.psize = &size;
  if (ease_in_active_ && ease_in_pos == 1.0f) {
    ease_in_active_ = false;
    ScheduleDismissal();
  }
  this->ease_in_pos_ = ease_in_pos;
  this->stack_collapse_pos_ = stack_collapse_pos;
  if (ease_out_pos != this->ease_out_pos_) {
    blend.BlendOp = AC_SRC_OVER;
    blend.BlendFlags = 0;
    blend.SourceConstantAlpha = (BYTE)(255 * (1.0f - ease_out_pos));
    blend.AlphaFormat = 0;
    ulw.pblend = &blend;
    ulw.dwFlags = ULW_ALPHA;
    this->ease_out_pos_ = ease_out_pos;
    if (ease_out_pos == 1.0f) {
      ease_out_active_ = false;
      dwpFlags &= ~SWP_SHOWWINDOW;
      dwpFlags |= SWP_HIDEWINDOW;
    }
  }
  if (stack_collapse_pos == 1.0f) {
    vertical_pos_ = vertical_pos_target_;
  }
  
  
  
  
  UpdateLayeredWindowIndirect(hwnd_, &ulw);
  hdwp = DeferWindowPos(hdwp, hwnd_, HWND_TOPMOST, pt.x, pt.y, size.cx, size.cy,