* 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
Emit("session-end");
}
void TopLevelWindow::OnWindowBlur() {
  EmitEventSoon("blur");
}
void TopLevelWindow::OnWindowFocus() {
  EmitEventSoon("focus");
}
void TopLevelWindow::OnWindowShow() {
  Emit("show");
}
void TopLevelWindow::OnWindowHide() {
  Emit("hide");
}
void TopLevelWindow::OnWindowMaximize() {
  Emit("maximize");
}
void TopLevelWindow::OnWindowUnmaximize() {
  Emit("unmaximize");
}
void TopLevelWindow::OnWindowMinimize() {