Tiny speed improvement in the perf test.

Bug: angleproject:3117
Change-Id: Ie2892772da4808615b2a810ff523602894440b64
Reviewed-on: https://chromium-review.googlesource.com/c/1350490Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>