Add support for building with MoltenVK

Bug: skia:
Change-Id: If55785d7fcc6e2c92c961ac390700add874c8d6d
Reviewed-on: https://skia-review.googlesource.com/125601
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed