* 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
auto handle = window_->GetNativeWindowHandlePointer();
  return ToBuffer(isolate(), std::get<0>(handle), std::get<1>(handle));
}
void TopLevelWindow::SetProgressBar(double progress, mate::Arguments* args) {
  mate::Dictionary options;
  std::string mode;
  args->GetNext(&options) && options.Get("mode", &mode);
  NativeWindow::ProgressState state = NativeWindow::PROGRESS_NORMAL;
  if (mode == "error")
    state = NativeWindow::PROGRESS_ERROR;
  else if (mode == "paused")
    state = NativeWindow::PROGRESS_PAUSED;
  else if (mode == "indeterminate")
    state = NativeWindow::PROGRESS_INDETERMINATE;
  else if (mode == "none")
    state = NativeWindow::PROGRESS_NONE;
  window_->SetProgressBar(progress, state);
}
void TopLevelWindow::SetOverlayIcon(const gfx::Image& overlay,