Merge "Revert "Revert "make it illegal to include SkXfermode.h""" into master-skia

https://skia.googlesource.com/skia/+/0e39f97cd7af16e57c7257770af8340750c9b3b0

Change-Id: Ia18edec4d0a91178b65f762a84e2f30ee18c1cae