Add a mock implementation of GrMeshDrawOp::Target

Moves the "BenchmarkTarget" class from tessellation benchmarks into a
mock header where it can be reused by other tests and benchmarks.

Change-Id: I344d9ba3d391ff99e10c4ab238684b0b6ada87d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333220
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed