Move SkLeanWindows.h to src.

This file pulls in Windows headers in a custom way, which is somewhat
awkward for a library header. The only use in include/ has been replaced
with a single forward declaration.

Change-Id: Ibef4cf7a2d1c9957a6a5b145b95aca1a6868cb5e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214689
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
24 files changed