* 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("isEnabled", &TopLevelWindow::IsEnabled)
      .SetMethod("setEnabled", &TopLevelWindow::SetEnabled)
      .SetMethod("maximize", &TopLevelWindow::Maximize)
      .SetMethod("unmaximize", &TopLevelWindow::Unmaximize)
      .SetMethod("isMaximized", &TopLevelWindow::IsMaximized)
      .SetMethod("minimize", &TopLevelWindow::Minimize)
      .SetMethod("restore", &TopLevelWindow::Restore)
      .SetMethod("isMinimized", &TopLevelWindow::IsMinimized)
      .SetMethod("setFullScreen", &TopLevelWindow::SetFullScreen)
      .SetMethod("isFullScreen", &TopLevelWindow::IsFullscreen)
      .SetMethod("setBounds", &TopLevelWindow::SetBounds)
      .SetMethod("getBounds", &TopLevelWindow::GetBounds)
      .SetMethod("isNormal", &TopLevelWindow::IsNormal)
      .SetMethod("getNormalBounds", &TopLevelWindow::GetNormalBounds)
      .SetMethod("setSize", &TopLevelWindow::SetSize)
      .SetMethod("getSize", &TopLevelWindow::GetSize)
      .SetMethod("setContentBounds", &TopLevelWindow::SetContentBounds)
      .SetMethod("getContentBounds", &TopLevelWindow::GetContentBounds)
      .SetMethod("setContentSize", &TopLevelWindow::SetContentSize)
      .SetMethod("getContentSize", &TopLevelWindow::GetContentSize)
      .SetMethod("setMinimumSize", &TopLevelWindow::SetMinimumSize)
      .SetMethod("getMinimumSize", &TopLevelWindow::GetMinimumSize)
      .SetMethod("setMaximumSize", &TopLevelWindow::SetMaximumSize)
      .SetMethod("getMaximumSize", &TopLevelWindow::GetMaximumSize)
      .SetMethod("setSheetOffset", &TopLevelWindow::SetSheetOffset)
      .SetMethod("setResizable", &TopLevelWindow::SetResizable)
      .SetMethod("isResizable", &TopLevelWindow::IsResizable)
      .SetMethod("setMovable", &TopLevelWindow::SetMovable)
#if defined(OS_WIN) || defined(OS_MACOSX)
      .SetMethod("moveTop", &TopLevelWindow::MoveTop)