move GrColor.h to private, IWYU

Bug: skia:
Change-Id: I0f0dabd7cc54cb7786f53bd6da0c0c012375037e
Reviewed-on: https://skia-review.googlesource.com/104160
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
20 files changed