Correct cluster index

Change-Id: Ic657b3fff318e1e76efa4ab7533bc2a833d17802
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253957
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
2 files changed