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
virtual void SetProgressBar(double progress) = 0;
  virtual void SetOverlayIcon(const gfx::Image& overlay,
                              const std::string& description) = 0;
  
  virtual void SetVisibleOnAllWorkspaces(bool visible) = 0;
  virtual bool IsVisibleOnAllWorkspaces() = 0;
  
  virtual void FocusOnWebView();
  virtual void BlurWebView();
  virtual bool IsWebViewFocused();
  
  virtual void SetAutoHideMenuBar(bool auto_hide);
  virtual bool IsMenuBarAutoHide();
  virtual void SetMenuBarVisibility(bool visible);
  virtual bool IsMenuBarVisible();
  
  double GetAspectRatio();
  gfx::Size GetAspectRatioExtraSize();
  virtual void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size);
  base::WeakPtr<NativeWindow> GetWeakPtr() {
    return weak_factory_.GetWeakPtr();
  }
  
  virtual void RequestToClosePage();
  
  virtual void CloseContents(content::WebContents* source);
  virtual void RendererUnresponsive(content::WebContents* source);
  virtual void RendererResponsive(content::WebContents* source);
  virtual void HandleKeyboardEvent(