Add missing const

Change-Id: If73c1b51bb484f76c7c25ab183995be11c3ffb23
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/759121
Commit-Queue: John Reck <jreck@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed