Make GrXferProcessor and related classes private.

Change-Id: I81ea6f5ea5c8b7b23848ef24524a7e48e531efe8
Reviewed-on: https://skia-review.googlesource.com/8819
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
7 files changed