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.pptDst = nullptr;
            ulw.psize = &toast_size_;
            ulw.hdcSrc = hdc_;
            ulw.pptSrc = &origin;
            ulw.crKey = 0;
            ulw.pblend = nullptr;
            ulw.dwFlags = 0;
            ulw.prcDirty = &dirty1;
            auto b1 = UpdateLayeredWindowIndirect(hwnd_, &ulw);
            ulw.prcDirty = &dirty2;
            auto b2 = UpdateLayeredWindowIndirect(hwnd_, &ulw);
            DCHECK(b1 && b2);
          }
          return;
        }
        DeleteBitmap(new_bitmap);
      }
    }
  }
}
void DesktopNotificationController::Toast::UpdateScaledImage(const SIZE& size) {
  BITMAP bm;
  if (!GetObject(scaled_image_, sizeof(bm), &bm) || bm.bmWidth != size.cx ||
      bm.bmHeight != size.cy) {
    if (scaled_image_)
      DeleteBitmap(scaled_image_);
    scaled_image_ = StretchBitmap(data_->image, size.cx, size.cy);
  }
}
void DesktopNotificationController::Toast::UpdateContents() {