[minor] tools/skottie_ios_app/Skia*Context: remember to cast

This will make a client builder happy.

Change-Id: Ia7b79768dc7ba9806d854740d1f7c4b2c02fa845
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264417
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed