Add a GpuGM class for GPU-only tests

Bug: skia:8731
Change-Id: Ic81f21621c0183bb356601eb663fa1ee334bca55
Reviewed-on: https://skia-review.googlesource.com/c/189134
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
23 files changed