make GrPaint.h private -- IWYU

prerequisit: https://codereview.chromium.org/2743533002/

BUG=skia:

Change-Id: I8c2f34a6df165744ddeaeabb562cd6d6a9679b6a
Reviewed-on: https://skia-review.googlesource.com/9461
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
9 files changed