Rename Benchmark::Backend::kGPU to Benchmark::Backend::kGanesh

This is a more accurate name. Also, we tend to prefer enum classes these
days, so this changes that as well.

Change-Id: I7bc194c1926dae1fd1edd76d7e42b8f3f4d646b3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/810358
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
77 files changed