Add placeholder GrMtlTypes in include/gpu/mtl

This is needed since there are currently no files in the include dir, and
some of our tests that check all the public headers complain if it doesn't
exists.

Bug: skia:6896
Change-Id: I3f70293b64b5096a55d6ba0ea5f4e6ebbfd7f62b
Reviewed-on: https://skia-review.googlesource.com/28003
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed