add fsync to try to get complete skps from webpage picture capture

R=reed@google.com,mtklein@google.com
BUG=skia:4691
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1530783003

Review URL: https://codereview.chromium.org/1530783003
4 files changed