pin before calling acos

Adobe reports some user crashes in acos(). While the cause is
unknown, it's safe and may help stability to pin the input
in case the arguments drifted slightly outside [-1, 1].

R=reed@google.com
BUG=skia:5222
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2006653006

Review-Url: https://codereview.chromium.org/2006653006
2 files changed