* 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
mate::Arguments* args) {
  if (IsModal()) {
    args->ThrowError("Can not be called for modal window");
    return;
  }
  mate::Handle<TopLevelWindow> parent;
  if (value->IsNull() || value->IsUndefined()) {
    RemoveFromParentChildWindows();
    parent_window_.Reset();
    window_->SetParentWindow(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &parent)) {
    parent_window_.Reset(isolate(), value);
    window_->SetParentWindow(parent->window_.get());
    parent->child_windows_.Set(isolate(), weak_map_id(), GetWrapper());
  } else {
    args->ThrowError("Must pass TopLevelWindow instance or null");
  }
}
void TopLevelWindow::SetBrowserView(v8::Local<v8::Value> value) {
  ResetBrowserView();
  mate::Handle<BrowserView> browser_view;
  if (value->IsNull() || value->IsUndefined()) {
    window_->SetBrowserView(nullptr);
  } else if (mate::ConvertFromV8(isolate(), value, &browser_view)) {
    window_->SetBrowserView(browser_view->view());
    browser_view->web_contents()->SetOwnerWindow(window_.get());
    browser_view_.Reset(isolate(), value);
  }
}
v8::Local<v8::Value> TopLevelWindow::GetNativeWindowHandle() {