GN: add skia_use_mesa for OSMesa in tools.

I tested by running
  $ out/dm --src gm --config mesa
  $ out/nanobench --config mesa --match Xfermode
... but I ran out of patience waiting for nanobench to finish.  Probably works.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2357703002

Review-Url: https://codereview.chromium.org/2357703002
3 files changed