[cleanup] rename var in perf script to match sktest script

NOTRY=true

Change-Id: Ib3c03092045289eb0828f951b1213ecab893d0d9
Reviewed-on: https://skia-review.googlesource.com/10969
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
1 file changed