GrSkSLFP no longer needs a GrContext

Change-Id: Iee1ea7ee2d545138a8243c373f7a163f7120548d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396337
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed