add virtual to confirm flattenable type

meant to replace https://skia-review.googlesource.com/c/skia/+/83381

Bug:793639
Change-Id: I972dc80725bf5ce5be24769aafa23269735e089c
Reviewed-on: https://skia-review.googlesource.com/83444
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed