* 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("minimize");
}
void TopLevelWindow::OnWindowRestore() {
  Emit("restore");
}
void TopLevelWindow::OnWindowWillResize(const gfx::Rect& new_bounds,
                                        bool* prevent_default) {
  if (Emit("will-resize", new_bounds)) {
    *prevent_default = true;
  }
}
void TopLevelWindow::OnWindowResize() {
  Emit("resize");
}
void TopLevelWindow::OnWindowWillMove(const gfx::Rect& new_bounds,
                                      bool* prevent_default) {
  if (Emit("will-move", new_bounds)) {
    *prevent_default = true;
  }
}
void TopLevelWindow::OnWindowMove() {
  Emit("move");
}
void TopLevelWindow::OnWindowMoved() {
  Emit("moved");
}
void TopLevelWindow::OnWindowEnterFullScreen() {
  Emit("enter-full-screen");
}
void TopLevelWindow::OnWindowLeaveFullScreen() {