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.h
observers_.AddObserver(obs);
  }
  void RemoveObserver(NativeWindowObserver* obs) {
    observers_.RemoveObserver(obs);
  }
  brightray::InspectableWebContents* inspectable_web_contents() const {
    return inspectable_web_contents_;
  }
  bool has_frame() const { return has_frame_; }
  void set_has_frame(bool has_frame) { has_frame_ = has_frame; }
  bool transparent() const { return transparent_; }
  SkRegion* draggable_region() const { return draggable_region_.get(); }
  bool enable_larger_than_screen() const { return enable_larger_than_screen_; }
  NativeWindow* parent() const { return parent_; }
  bool is_modal() const { return is_modal_; }
 protected:
  NativeWindow(brightray::InspectableWebContents* inspectable_web_contents,
               const mate::Dictionary& options,
               NativeWindow* parent);
  
  
  std::unique_ptr<SkRegion> DraggableRegionsToSkRegion(
      const std::vector<DraggableRegion>& regions);
  
  virtual gfx::Size ContentSizeToWindowSize(const gfx::Size& size) = 0;
  virtual gfx::Size WindowSizeToContentSize(const gfx::Size& size) = 0;
  
  virtual void UpdateDraggableRegions(
      const std::vector<DraggableRegion>& regions);