skpbench: add option for gpu timing

Adds a gpu timing option with a GL implementation.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2388433003

Committed: https://skia.googlesource.com/skia/+/c06720d06faab3b01eba1b8693e0ac791f06dc96
Review-Url: https://codereview.chromium.org/2388433003
9 files changed