Graphite: Add stubs for Pipeline and PipelineDesc

Bug: skia:12466
Change-Id: Ie949a17166b3ab5c529b74a6762c8a6f8ca8052f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453061
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
7 files changed