* 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)
  
  if (!(::GetWindowLong(GetAcceleratedWidget(), GWL_STYLE) & WS_THICKFRAME)) {
    restore_bounds_ = GetBounds();
    auto display =
        display::Screen::GetScreen()->GetDisplayNearestPoint(GetPosition());
    SetBounds(display.work_area(), false);
    return;
  }
#endif
  if (IsVisible())
    widget()->Maximize();
  else
    widget()->native_widget_private()->Show(ui::SHOW_STATE_MAXIMIZED,
                                            gfx::Rect());
}
void NativeWindowViews::Unmaximize() {