skpbench: use 3 cores instead of 4 on Pixel C

Hopefully with less cores running we will produce less heat. 3 cores
should be enough to run Ganesh, the graphics driver, and the OS.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2491633002

Review-Url: https://codereview.chromium.org/2491633002
1 file changed