Turn on MPD threading in nanobench.

Seems okay after this small patch to skip lockPixels() / unlockPixels().

BUG=skia:3149

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

Review URL: https://codereview.chromium.org/773203003
2 files changed