flush + fsync the skp after it is written
flush + fsync the skp after it is written

Add a explicit flush() and fsync() before the SkFILEWStream
destructor is called.

Hopefully this will eliminate truncated SKPs generated by Skia's
infra team when capturing the drawing on the top 10K / 1M websites.

R=alokp@chromium.org,mtklein@chromium.org
BUG=569890,skia:4691

Review URL: https://codereview.chromium.org/1527943002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365917}

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