* 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 defined(OS_WIN)
  if (has_frame())
    return ::GetWindowLong(GetAcceleratedWidget(), GWL_STYLE) & WS_THICKFRAME;
#endif
  return CanResize();
}
void NativeWindowViews::SetMovable(bool movable) {
  movable_ = movable;
}
bool NativeWindowViews::IsMovable() {
#if defined(OS_WIN)
  return movable_;
#else
  return true;  
#endif
}
void NativeWindowViews::SetMinimizable(bool minimizable) {
#if defined(OS_WIN)
  FlipWindowStyle(GetAcceleratedWidget(), minimizable, WS_MINIMIZEBOX);
#endif
  minimizable_ = minimizable;
}
bool NativeWindowViews::IsMinimizable() {
#if defined(OS_WIN)
  return ::GetWindowLong(GetAcceleratedWidget(), GWL_STYLE) & WS_MINIMIZEBOX;
#else
  return true;  
#endif
}
void NativeWindowViews::SetMaximizable(bool maximizable) {