* 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/native_window_views.cc
if (options.Get(options::kDarkTheme, &use_dark_theme) && use_dark_theme) {
    XDisplay* xdisplay = gfx::GetXDisplay();
    XChangeProperty(xdisplay, GetAcceleratedWidget(),
                    XInternAtom(xdisplay, "_GTK_THEME_VARIANT", x11::False),
                    XInternAtom(xdisplay, "UTF8_STRING", x11::False), 8,
                    PropModeReplace,
                    reinterpret_cast<const unsigned char*>("dark"), 4);
  }
  
  
  std::vector<::Atom> state_atom_list;
  bool skip_taskbar = false;
  if (options.Get(options::kSkipTaskbar, &skip_taskbar) && skip_taskbar) {
    state_atom_list.push_back(GetAtom("_NET_WM_STATE_SKIP_TASKBAR"));
  }
  
  if (fullscreen) {
    state_atom_list.push_back(GetAtom("_NET_WM_STATE_FULLSCREEN"));
  }
  if (parent) {
    SetParentWindow(parent);
    
    window_type = "dialog";
    
    if (is_modal())
      state_atom_list.push_back(GetAtom("_NET_WM_STATE_MODAL"));
  }
  if (!state_atom_list.empty())
    ui::SetAtomArrayProperty(GetAcceleratedWidget(), "_NET_WM_STATE", "ATOM",
                             state_atom_list);