* 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 (resizable != resizable_) {
    
    
    if (resizable) {
      SetContentSizeConstraints(old_size_constraints_);
      SetMaximizable(maximizable_);
    } else {
      old_size_constraints_ = GetContentSizeConstraints();
      resizable_ = false;
      gfx::Size content_size = GetContentSize();
      SetContentSizeConstraints(
          extensions::SizeConstraints(content_size, content_size));
    }
  }
#if defined(OS_WIN)
  if (has_frame() && thick_frame_)
    FlipWindowStyle(GetAcceleratedWidget(), resizable, WS_THICKFRAME);
#endif
  resizable_ = resizable;
}
#if defined(OS_WIN)
void NativeWindowViews::MoveTop() {
  gfx::Point pos = GetPosition();
  gfx::Size size = GetSize();
  ::SetWindowPos(GetAcceleratedWidget(), HWND_TOP, pos.x(), pos.y(),
                 size.width(), size.height(),
                 SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE | SWP_SHOWWINDOW);
}
#endif
bool NativeWindowViews::IsResizable() {