Small performance optimization.

Bug: angleproject:2786
Change-Id: Ib01c1762d438ffb72ec25bf05135a7d3271d6aee
Reviewed-on: https://chromium-review.googlesource.com/1188954
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>