* Output of `node_modules/.bin/electron --version`: 4.0.0-beta.8
* Operating System (Platform and Version): Windows 10 1803
* Output of `node_modules/.bin/electron --version` on last known working Electron version (if applicable): 4.0.0-beta.7

**Expected Behavior**
`BrowserWindow.getNativeWindowHandle()` should return different value for different window

**Actual behavior**
`BrowserWindow.getNativeWindowHandle()` returns same value for different window, which is not true nativeWindowHandle for any of them.

**To Reproduce**
Open two `BrowserWindow`, and run `getNativeWindowHandle` on both `BrowserWindow`
```js
const { remote } = require('electron')
w1 = new remote.BrowserWindow()
w2 = new remote.BrowserWindow()
console.log(w1.getNativeWindowHandle(), w2.getNativeWindowHandle())
```
![image](https://user-images.githubusercontent.com/8194131/49328509-7411a800-f5b5-11e8-9a24-827aa589d937.png)


atom/browser/native_window_views.cc
#if defined(OS_WIN)
  Microsoft::WRL::ComPtr<ITaskbarList> taskbar;
  if (FAILED(::CoCreateInstance(CLSID_TaskbarList, nullptr,
                                CLSCTX_INPROC_SERVER,
                                IID_PPV_ARGS(&taskbar))) ||
      FAILED(taskbar->HrInit()))
    return;
  if (skip) {
    taskbar->DeleteTab(GetAcceleratedWidget());
  } else {
    taskbar->AddTab(GetAcceleratedWidget());
    taskbar_host_.RestoreThumbarButtons(GetAcceleratedWidget());
  }
#elif defined(USE_X11)
  SetWMSpecState(GetAcceleratedWidget(), skip,
                 GetAtom("_NET_WM_STATE_SKIP_TASKBAR"));
#endif
}
void NativeWindowViews::SetSimpleFullScreen(bool simple_fullscreen) {