As a new year gift I have implemented the Windows version of SDL_GetWindowBordersSize(). I needed it for auto-selecting a cozy window size for the game I’m currently working on and noticed that it only worked under X11/Linux, so I thought it could be a good excuse to contribute back more stuff.
The Mercurial patch is attached as a .diff file. Let me know what you think.
Happy 2018 to all the SDL2 devs and users! 
Here is the Bugzilla ticket with the attached patch: https://bugzilla.libsdl.org/show_bug.cgi?id=4018