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.cc
int x = -1, y = -1;
  bool center;
  if (options.Get(options::kX, &x) && options.Get(options::kY, &y)) {
    SetPosition(gfx::Point(x, y));
  } else if (options.Get(options::kCenter, &center) && center) {
    Center();
  }
  
  extensions::SizeConstraints size_constraints(GetContentSizeConstraints());
  int min_height = 0, min_width = 0;
  if (options.Get(options::kMinHeight, &min_height) |
      options.Get(options::kMinWidth, &min_width)) {
    size_constraints.set_minimum_size(gfx::Size(min_width, min_height));
  }
  int max_height = INT_MAX, max_width = INT_MAX;
  if (options.Get(options::kMaxHeight, &max_height) |
      options.Get(options::kMaxWidth, &max_width)) {
    size_constraints.set_maximum_size(gfx::Size(max_width, max_height));
  }
  bool use_content_size = false;
  options.Get(options::kUseContentSize, &use_content_size);
  if (use_content_size) {
    SetContentSizeConstraints(size_constraints);
  } else {
    SetSizeConstraints(size_constraints);
  }
#if defined(USE_X11)
  bool resizable;
  if (options.Get(options::kResizable, &resizable)) {