Force SkPath::getConvexity() cache for thread safety.

I happened to stumble on this in a run of TSAN:
    https://luci-milo.appspot.com/swarming/task/30fffe9497dc6310/steps/dm/0/stdout

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2300003003

Review-Url: https://codereview.chromium.org/2300003003
2 files changed