Wrap GrMtlRenderCommandEncoder methods in AVAILABLE checks

Change-Id: I8d165b2d9b77b83ff32d8f081efdd927ab8d1aba
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/418737
Auto-Submit: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed