[canvaskit] Correctly translate between shadow radius and sigma

Bug: skia:
Change-Id: I7e069407334f8b0238fd6d9f529cdf30e3e269be
Reviewed-on: https://skia-review.googlesource.com/c/176960
Reviewed-by: Florin Malita <fmalita@chromium.org>
1 file changed