* 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
const gfx::Rect& bounds) {
  if (changed_widget != widget())
    return;
  
  
  const auto new_bounds = GetBounds();
  if (widget_size_ != new_bounds.size()) {
    if (browser_view()) {
      const auto flags = static_cast<NativeBrowserViewViews*>(browser_view())
                             ->GetAutoResizeFlags();
      int width_delta = 0;
      int height_delta = 0;
      if (flags & kAutoResizeWidth) {
        width_delta = new_bounds.width() - widget_size_.width();
      }
      if (flags & kAutoResizeHeight) {
        height_delta = new_bounds.height() - widget_size_.height();
      }
      auto* view = browser_view()->GetInspectableWebContentsView()->GetView();
      auto new_view_size = view->size();
      new_view_size.set_width(new_view_size.width() + width_delta);
      new_view_size.set_height(new_view_size.height() + height_delta);
      view->SetSize(new_view_size);
    }
    NotifyWindowResize();
    widget_size_ = new_bounds.size();
  }
}
void NativeWindowViews::DeleteDelegate() {