skpbench: use double buffering instead of triple

We have no need for our command stream to get more than a couple of
frames deep. Any more and we only increase our chances of sporadic
flushing and unreliable timings.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2376043002

Review-Url: https://codereview.chromium.org/2376043002
1 file changed