* 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
widget()->CenterWindow(GetSize());
}
void NativeWindowViews::Invalidate() {
  widget()->SchedulePaintInRect(gfx::Rect(GetBounds().size()));
}
void NativeWindowViews::SetTitle(const std::string& title) {
  title_ = title;
  widget()->UpdateWindowTitle();
}
std::string NativeWindowViews::GetTitle() {
  return title_;
}
void NativeWindowViews::FlashFrame(bool flash) {
#if defined(OS_WIN)
  
  if (!flash) {
    FLASHWINFO fwi;
    fwi.cbSize = sizeof(fwi);
    fwi.hwnd = GetAcceleratedWidget();
    fwi.dwFlags = FLASHW_STOP;
    fwi.uCount = 0;
    FlashWindowEx(&fwi);
    return;
  }
#endif
  widget()->FlashFrame(flash);
}
void NativeWindowViews::SetSkipTaskbar(bool skip) {