Allow mock GrContexts to "support" shader derivatives

Change-Id: I0d8dce5e0873a058b59baae9c35903b2444a76fd
Reviewed-on: https://skia-review.googlesource.com/68480
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed