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_mac.h
#ifndef ATOM_BROWSER_NATIVE_WINDOW_MAC_H_
#define ATOM_BROWSER_NATIVE_WINDOW_MAC_H_
#import <Cocoa/Cocoa.h>
#include <string>
#include <vector>
#include "base/mac/scoped_nsobject.h"
#include "atom/browser/native_window.h"
@class AtomNSWindow;