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This extra atomic bool is hard to think about, and I'm worried about how
updates to fGenerationID and fUniqueGenerationID interlace.  By storing
them in the same int, they can't ever race.

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu13.10-GCE-NoGPU-x86_64-Release-TSAN-Trybot

BUG=skia:

Review URL: https://codereview.chromium.org/955043002