* 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
#endif
      .SetMethod("isMovable", &TopLevelWindow::IsMovable)
      .SetMethod("setMinimizable", &TopLevelWindow::SetMinimizable)
      .SetMethod("isMinimizable", &TopLevelWindow::IsMinimizable)
      .SetMethod("setMaximizable", &TopLevelWindow::SetMaximizable)
      .SetMethod("isMaximizable", &TopLevelWindow::IsMaximizable)
      .SetMethod("setFullScreenable", &TopLevelWindow::SetFullScreenable)
      .SetMethod("isFullScreenable", &TopLevelWindow::IsFullScreenable)
      .SetMethod("setClosable", &TopLevelWindow::SetClosable)
      .SetMethod("isClosable", &TopLevelWindow::IsClosable)
      .SetMethod("setAlwaysOnTop", &TopLevelWindow::SetAlwaysOnTop)
      .SetMethod("isAlwaysOnTop", &TopLevelWindow::IsAlwaysOnTop)
      .SetMethod("center", &TopLevelWindow::Center)
      .SetMethod("setPosition", &TopLevelWindow::SetPosition)
      .SetMethod("getPosition", &TopLevelWindow::GetPosition)
      .SetMethod("setTitle", &TopLevelWindow::SetTitle)
      .SetMethod("getTitle", &TopLevelWindow::GetTitle)
      .SetMethod("flashFrame", &TopLevelWindow::FlashFrame)
      .SetMethod("setSkipTaskbar", &TopLevelWindow::SetSkipTaskbar)
      .SetMethod("setSimpleFullScreen", &TopLevelWindow::SetSimpleFullScreen)
      .SetMethod("isSimpleFullScreen", &TopLevelWindow::IsSimpleFullScreen)
      .SetMethod("setKiosk", &TopLevelWindow::SetKiosk)
      .SetMethod("isKiosk", &TopLevelWindow::IsKiosk)
      .SetMethod("setBackgroundColor", &TopLevelWindow::SetBackgroundColor)
      .SetMethod("setHasShadow", &TopLevelWindow::SetHasShadow)
      .SetMethod("hasShadow", &TopLevelWindow::HasShadow)
      .SetMethod("setOpacity", &TopLevelWindow::SetOpacity)
      .SetMethod("getOpacity", &TopLevelWindow::GetOpacity)
      .SetMethod("setShape", &TopLevelWindow::SetShape)
      .SetMethod("setRepresentedFilename",