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
diff --git a/include/core/SkStream.h b/include/core/SkStream.h
index 44de6e8..4502416 100644
--- a/include/core/SkStream.h
+++ b/include/core/SkStream.h
@@ -360,6 +360,7 @@
 
     bool write(const void* buffer, size_t size) override;
     void flush() override;
+    void fsync();
     size_t bytesWritten() const override;
 
 private: