Adds a prebuilt MoltenVK framework asset from:
KhronosGroup/MoltenVK.git:0279543e0bb561f8d0166d4030481136bc914fec
And creates bots that use it.
Bug: skia:
Change-Id: I081b2745c41a25de9ecd94a5363f4fb13b120215
Reviewed-on: https://skia-review.googlesource.com/128646
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
