* 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 (is_modal() && this->parent()) {
    auto* parent = this->parent();
    
    static_cast<NativeWindowViews*>(parent)->DecrementChildModals();
    
    parent->Focus(true);
  }
  NotifyWindowClosed();
}
views::View* NativeWindowViews::GetInitiallyFocusedView() {
  return focused_view_;
}
bool NativeWindowViews::CanResize() const {
  return resizable_;
}
bool NativeWindowViews::CanMaximize() const {
  return resizable_ && maximizable_;
}
bool NativeWindowViews::CanMinimize() const {