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
std::string GetRepresentedFilename() override;
  void SetDocumentEdited(bool edited) override;
  bool IsDocumentEdited() override;
  void SetIgnoreMouseEvents(bool ignore) override;
  void SetContentProtection(bool enable) override;
  void SetParentWindow(NativeWindow* parent) override;
  gfx::NativeWindow GetNativeWindow() override;
  gfx::AcceleratedWidget GetAcceleratedWidget() override;
  void SetProgressBar(double progress) override;
  void SetOverlayIcon(const gfx::Image& overlay,
                      const std::string& description) override;
  void SetVisibleOnAllWorkspaces(bool visible) override;
  bool IsVisibleOnAllWorkspaces() override;
  
  void UpdateDraggableRegionViews() {
    UpdateDraggableRegionViews(draggable_regions_);
  }
  
  void SetStyleMask(bool on, NSUInteger flag);
  void SetCollectionBehavior(bool on, NSUInteger flag);
  enum TitleBarStyle {
    NORMAL,
    HIDDEN,
    HIDDEN_INSET,
  };
  TitleBarStyle title_bar_style() const { return title_bar_style_; }
 protected:
  
  
  std::vector<gfx::Rect> CalculateNonDraggableRegions(