* 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_->Unmaximize();
}
bool TopLevelWindow::IsMaximized() {
  return window_->IsMaximized();
}
void TopLevelWindow::Minimize() {
  window_->Minimize();
}
void TopLevelWindow::Restore() {
  window_->Restore();
}
bool TopLevelWindow::IsMinimized() {
  return window_->IsMinimized();
}
void TopLevelWindow::SetFullScreen(bool fullscreen) {
  window_->SetFullScreen(fullscreen);
}
bool TopLevelWindow::IsFullscreen() {
  return window_->IsFullscreen();
}
void TopLevelWindow::SetBounds(const gfx::Rect& bounds, mate::Arguments* args) {
  bool animate = false;
  args->GetNext(&animate);
  window_->SetBounds(bounds, animate);
}
gfx::Rect TopLevelWindow::GetBounds() {
  return window_->GetBounds();
}
bool TopLevelWindow::IsNormal() {