Hide GrFillRRectOp's implementation

This brings this op into closer correspondence with the other ops.

Change-Id: Idf4f68dcad5492d9ae66c34a9c8a86ff925de26e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/273492
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed