Fix race condition in SkROBuffer.

SkBufferBlock::fUsed may be updated by the writer while a reader is
attempting to read it.

BUG=chromium:601578
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1872853002

Review URL: https://codereview.chromium.org/1872853002
3 files changed