Make GrMesh compatible with instanced draws that have no attribs

Bug: skia:
Change-Id: Ia366e20608670df34ab2db3966f5beaa23242592
Reviewed-on: https://skia-review.googlesource.com/c/196162
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed