* 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_->SetHasShadow(has_shadow);
}
bool TopLevelWindow::HasShadow() {
  return window_->HasShadow();
}
void TopLevelWindow::SetOpacity(const double opacity) {
  window_->SetOpacity(opacity);
}
double TopLevelWindow::GetOpacity() {
  return window_->GetOpacity();
}
void TopLevelWindow::SetShape(const std::vector<gfx::Rect>& rects) {
  window_->widget()->SetShape(std::make_unique<std::vector<gfx::Rect>>(rects));
}
void TopLevelWindow::SetRepresentedFilename(const std::string& filename) {
  window_->SetRepresentedFilename(filename);
}
std::string TopLevelWindow::GetRepresentedFilename() {
  return window_->GetRepresentedFilename();
}
void TopLevelWindow::SetDocumentEdited(bool edited) {
  window_->SetDocumentEdited(edited);
}
bool TopLevelWindow::IsDocumentEdited() {
  return window_->IsDocumentEdited();
}
void TopLevelWindow::SetIgnoreMouseEvents(bool ignore, mate::Arguments* args) {