* 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
window_->SetTitle(title);
}
std::string TopLevelWindow::GetTitle() {
  return window_->GetTitle();
}
void TopLevelWindow::FlashFrame(bool flash) {
  window_->FlashFrame(flash);
}
void TopLevelWindow::SetSkipTaskbar(bool skip) {
  window_->SetSkipTaskbar(skip);
}
void TopLevelWindow::SetSimpleFullScreen(bool simple_fullscreen) {
  window_->SetSimpleFullScreen(simple_fullscreen);
}
bool TopLevelWindow::IsSimpleFullScreen() {
  return window_->IsSimpleFullScreen();
}
void TopLevelWindow::SetKiosk(bool kiosk) {
  window_->SetKiosk(kiosk);
}
bool TopLevelWindow::IsKiosk() {
  return window_->IsKiosk();
}
void TopLevelWindow::SetBackgroundColor(const std::string& color_name) {
  SkColor color = ParseHexColor(color_name);
  window_->SetBackgroundColor(color);
}
void TopLevelWindow::SetHasShadow(bool has_shadow) {