Add missing optional include.

Change-Id: I9c0dd750665a11a0329c2ca2455390ca4f21ad91
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1205716
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Auto-Submit: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>