Fix FrontBufferedStreamTest

TBR=scroggo@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1747423002

Review URL: https://codereview.chromium.org/1747423002
1 file changed