Fix shader compilation error in GrInstancedArraysBench

Change-Id: Ifd87e752fe1712ab4adef6c5f5de8798ab6c4991
Reviewed-on: https://skia-review.googlesource.com/50680
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed