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
const POINT& origin) {
  UpdateBufferSize();
  if (IsRedrawNeeded())
    Draw();
  POINT src_origin = {0, 0};
  UPDATELAYEREDWINDOWINFO ulw;
  ulw.cbSize = sizeof(ulw);
  ulw.hdcDst = NULL;
  ulw.pptDst = nullptr;
  ulw.psize = nullptr;
  ulw.hdcSrc = hdc_;
  ulw.pptSrc = &src_origin;
  ulw.crKey = 0;
  ulw.pblend = nullptr;
  ulw.dwFlags = 0;
  ulw.prcDirty = nullptr;
  POINT pt = {0, 0};
  SIZE size = {0, 0};
  BLENDFUNCTION blend;
  UINT dwpFlags =
      SWP_NOACTIVATE | SWP_SHOWWINDOW | SWP_NOREDRAW | SWP_NOCOPYBITS;
  auto ease_in_pos = AnimateEaseIn();
  auto ease_out_pos = AnimateEaseOut();
  auto stack_collapse_pos = AnimateStackCollapse();
  auto y_offset = (vertical_pos_target_ - vertical_pos_) * stack_collapse_pos;
  size.cx = static_cast<int>(toast_size_.cx * ease_in_pos);
  size.cy = toast_size_.cy;
  pt.x = origin.x - size.cx;
  pt.y = static_cast<int>(origin.y - vertical_pos_ - y_offset - size.cy);
  ulw.pptDst = &pt;