* 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
.SetMethod("selectPreviousTab", &TopLevelWindow::SelectPreviousTab)
      .SetMethod("selectNextTab", &TopLevelWindow::SelectNextTab)
      .SetMethod("mergeAllWindows", &TopLevelWindow::MergeAllWindows)
      .SetMethod("moveTabToNewWindow", &TopLevelWindow::MoveTabToNewWindow)
      .SetMethod("toggleTabBar", &TopLevelWindow::ToggleTabBar)
      .SetMethod("addTabbedWindow", &TopLevelWindow::AddTabbedWindow)
      .SetMethod("setWindowButtonVisibility",
                 &TopLevelWindow::SetWindowButtonVisibility)
#endif
      .SetMethod("setAutoHideMenuBar", &TopLevelWindow::SetAutoHideMenuBar)
      .SetMethod("isMenuBarAutoHide", &TopLevelWindow::IsMenuBarAutoHide)
      .SetMethod("setMenuBarVisibility", &TopLevelWindow::SetMenuBarVisibility)
      .SetMethod("isMenuBarVisible", &TopLevelWindow::IsMenuBarVisible)
      .SetMethod("setAspectRatio", &TopLevelWindow::SetAspectRatio)
      .SetMethod("previewFile", &TopLevelWindow::PreviewFile)
      .SetMethod("closeFilePreview", &TopLevelWindow::CloseFilePreview)
      .SetMethod("getContentView", &TopLevelWindow::GetContentView)
      .SetMethod("getParentWindow", &TopLevelWindow::GetParentWindow)
      .SetMethod("getChildWindows", &TopLevelWindow::GetChildWindows)
      .SetMethod("getBrowserView", &TopLevelWindow::GetBrowserView)
      .SetMethod("isModal", &TopLevelWindow::IsModal)
      .SetMethod("setThumbarButtons", &TopLevelWindow::SetThumbarButtons)
#if defined(TOOLKIT_VIEWS)
      .SetMethod("setIcon", &TopLevelWindow::SetIcon)
#endif
#if defined(OS_WIN)
      .SetMethod("hookWindowMessage", &TopLevelWindow::HookWindowMessage)
      .SetMethod("isWindowMessageHooked",
                 &TopLevelWindow::IsWindowMessageHooked)
      .SetMethod("unhookWindowMessage", &TopLevelWindow::UnhookWindowMessage)