* 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/api/atom_api_top_level_window.cc
#if defined(OS_WIN)
namespace mate {
template <>
struct Converter<atom::TaskbarHost::ThumbarButton> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Handle<v8::Value> val,
                     atom::TaskbarHost::ThumbarButton* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    dict.Get("click", &(out->clicked_callback));
    dict.Get("tooltip", &(out->tooltip));
    dict.Get("flags", &out->flags);
    return dict.Get("icon", &(out->icon));
  }
};
}  
#endif
namespace atom {
namespace api {
namespace {
v8::Local<v8::Value> ToBuffer(v8::Isolate* isolate, void* val, int size) {
  auto buffer = node::Buffer::Copy(isolate, static_cast<char*>(val), size);
  if (buffer.IsEmpty())
    return v8::Null(isolate);
  else
    return buffer.ToLocalChecked();
}
}  
TopLevelWindow::TopLevelWindow(v8::Isolate* isolate,