As in topic. API currently offers fixed ratio only for Mac but not for Windows
shell/browser/ui/views/native_frame_view.cc
#include "shell/browser/ui/views/native_frame_view.h"
#include "shell/browser/native_window.h"
namespace electron {
const char NativeFrameView::kViewClassName[] = "ElectronNativeFrameView";
NativeFrameView::NativeFrameView(NativeWindow* window, views::Widget* widget)
    : views::NativeFrameView(widget), window_(window) {}
gfx::Size NativeFrameView::GetMinimumSize() const {
  return window_->GetMinimumSize();
}
gfx::Size NativeFrameView::GetMaximumSize() const {
  return window_->GetMaximumSize();
}
const char* NativeFrameView::GetClassName() const {
  return kViewClassName;
}
}