[canvaskit] Fix bug if webgl context not available

Bug: skia:11107
Change-Id: Ibf697a2c0b471ea79bee1a33d8230086509147ca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/347677
Reviewed-by: Nathaniel Nifong <nifong@google.com>
2 files changed