First implementation of GrDawnVaryingHandler.

Change-Id: I49a233a28d16622b286ffc4150446b33f899a891
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230557
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
3 files changed