* 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 (event->type() != ui::ET_MOUSE_PRESSED)
    return;
  if (event->changed_button_flags() == ui::EF_BACK_MOUSE_BUTTON)
    NotifyWindowExecuteAppCommand(kBrowserBackward);
  else if (event->changed_button_flags() == ui::EF_FORWARD_MOUSE_BUTTON)
    NotifyWindowExecuteAppCommand(kBrowserForward);
}
#endif
ui::WindowShowState NativeWindowViews::GetRestoredState() {
  if (IsMaximized())
    return ui::SHOW_STATE_MAXIMIZED;
  if (IsFullscreen())
    return ui::SHOW_STATE_FULLSCREEN;
  return ui::SHOW_STATE_NORMAL;
}
NativeWindow* NativeWindow::Create(const mate::Dictionary& options,
                                   NativeWindow* parent) {
  return new NativeWindowViews(options, parent);
}
}