* 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
.SetMethod("unhookAllWindowMessages",
                 &TopLevelWindow::UnhookAllWindowMessages)
      .SetMethod("setThumbnailClip", &TopLevelWindow::SetThumbnailClip)
      .SetMethod("setThumbnailToolTip", &TopLevelWindow::SetThumbnailToolTip)
      .SetMethod("setAppDetails", &TopLevelWindow::SetAppDetails)
#endif
      .SetProperty("id", &TopLevelWindow::GetID);
}
}  
}  
namespace {
using atom::api::TopLevelWindow;
void Initialize(v8::Local<v8::Object> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  TopLevelWindow::SetConstructor(isolate, base::Bind(&TopLevelWindow::New));
  mate::Dictionary constructor(
      isolate, TopLevelWindow::GetConstructor(isolate)->GetFunction());
  constructor.SetMethod("fromId",
                        &mate::TrackableObject<TopLevelWindow>::FromWeakMapID);
  constructor.SetMethod("getAllWindows",
                        &mate::TrackableObject<TopLevelWindow>::GetAll);
  mate::Dictionary dict(isolate, exports);
  dict.Set("TopLevelWindow", constructor);
}
}  
NODE_BUILTIN_MODULE_CONTEXT_AWARE(atom_browser_top_level_window, Initialize)