* 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
&TopLevelWindow::SetRepresentedFilename)
      .SetMethod("getRepresentedFilename",
                 &TopLevelWindow::GetRepresentedFilename)
      .SetMethod("setDocumentEdited", &TopLevelWindow::SetDocumentEdited)
      .SetMethod("isDocumentEdited", &TopLevelWindow::IsDocumentEdited)
      .SetMethod("setIgnoreMouseEvents", &TopLevelWindow::SetIgnoreMouseEvents)
      .SetMethod("setContentProtection", &TopLevelWindow::SetContentProtection)
      .SetMethod("setFocusable", &TopLevelWindow::SetFocusable)
      .SetMethod("setMenu", &TopLevelWindow::SetMenu)
#if !defined(OS_WIN)
      .SetMethod("setParentWindow", &TopLevelWindow::SetParentWindow)
#endif
      .SetMethod("setBrowserView", &TopLevelWindow::SetBrowserView)
      .SetMethod("getNativeWindowHandle",
                 &TopLevelWindow::GetNativeWindowHandle)
      .SetMethod("setProgressBar", &TopLevelWindow::SetProgressBar)
      .SetMethod("setOverlayIcon", &TopLevelWindow::SetOverlayIcon)
      .SetMethod("setVisibleOnAllWorkspaces",
                 &TopLevelWindow::SetVisibleOnAllWorkspaces)
      .SetMethod("isVisibleOnAllWorkspaces",
                 &TopLevelWindow::IsVisibleOnAllWorkspaces)
#if defined(OS_MACOSX)
      .SetMethod("setAutoHideCursor", &TopLevelWindow::SetAutoHideCursor)
#endif
      .SetMethod("setVibrancy", &TopLevelWindow::SetVibrancy)
      .SetMethod("_setTouchBarItems", &TopLevelWindow::SetTouchBar)
      .SetMethod("_refreshTouchBarItem", &TopLevelWindow::RefreshTouchBarItem)
      .SetMethod("_setEscapeTouchBarItem",
                 &TopLevelWindow::SetEscapeTouchBarItem)
#if defined(OS_MACOSX)