* 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("leave-full-screen");
}
void TopLevelWindow::OnWindowScrollTouchBegin() {
  Emit("scroll-touch-begin");
}
void TopLevelWindow::OnWindowScrollTouchEnd() {
  Emit("scroll-touch-end");
}
void TopLevelWindow::OnWindowSwipe(const std::string& direction) {
  Emit("swipe", direction);
}
void TopLevelWindow::OnWindowSheetBegin() {
  Emit("sheet-begin");
}
void TopLevelWindow::OnWindowSheetEnd() {
  Emit("sheet-end");
}
void TopLevelWindow::OnWindowEnterHtmlFullScreen() {
  Emit("enter-html-full-screen");
}
void TopLevelWindow::OnWindowLeaveHtmlFullScreen() {
  Emit("leave-html-full-screen");
}
void TopLevelWindow::OnWindowAlwaysOnTopChanged() {
  Emit("always-on-top-changed", IsAlwaysOnTop());
}
void TopLevelWindow::OnExecuteAppCommand(const std::string& command_name) {
  Emit("app-command", command_name);
}
void TopLevelWindow::OnTouchBarItemResult(