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
extensions::SizeConstraints size_constraints;
  size_constraints.set_maximum_size(size);
  SetSizeConstraints(size_constraints);
}
gfx::Size NativeWindow::GetMaximumSize() {
  return GetSizeConstraints().GetMaximumSize();
}
void NativeWindow::SetSheetOffset(const double offsetX, const double offsetY) {
  sheet_offset_x_ = offsetX;
  sheet_offset_y_ = offsetY;
}
double NativeWindow::GetSheetOffsetX() {
  return sheet_offset_x_;
}
double NativeWindow::GetSheetOffsetY() {
  return sheet_offset_y_;
}
void NativeWindow::SetRepresentedFilename(const std::string& filename) {
}
std::string NativeWindow::GetRepresentedFilename() {
  return "";
}
void NativeWindow::SetDocumentEdited(bool edited) {
}
bool NativeWindow::IsDocumentEdited() {
  return false;
}
void NativeWindow::SetFocusable(bool focusable) {