We can get the content size and the window size, but not the chrome position.

At the moment we do this:

```
leftBorderWidth = (windowWidth - contentWidth) / 2;
titlebarHeight = windowHeight - (contentHeight + leftBorderWidth);
```

which makes some assumptions so therefore feels a bit wrong.

it would be nice if alongside getContentSize (https://github.com/electron/electron/blob/master/docs/api/browser-window.md#wingetcontentsize) there was a getContentOffset.

I've been looking at the C++ - https://github.com/electron/electron/blob/e05804848f98959d63337aacd9047cd579c5b23f/chromium_src/extensions/browser/app_window/size_constraints.cc and I don't see an easy way to get this information - do you have any ideas or pointers if you accept this as a feature request?

atom/browser/native_window_views.cc
#if defined(OS_WIN)
  if (IsMinimized())
    return window_->GetRestoredBounds();
#endif
  return window_->GetWindowBoundsInScreen();
}
gfx::Size NativeWindowViews::GetContentSize() {
#if defined(OS_WIN)
  if (IsMinimized())
    return NativeWindow::GetContentSize();
#endif
  return web_view_->size();
}
void NativeWindowViews::SetContentSizeConstraints(
    const extensions::SizeConstraints& size_constraints) {
  NativeWindow::SetContentSizeConstraints(size_constraints);
  
  
  if (window_ && window_->widget_delegate())
    window_->OnSizeConstraintsChanged();
#if defined(USE_X11)
  if (resizable_)
    old_size_constraints_ = size_constraints;
#endif
}
void NativeWindowViews::SetResizable(bool resizable) {