roll libyuv from r1515 to r1521
roll libyuv from r1515 to r1521

This roll contains fixes for arm unittest failures for cpus without Neon,
and for odd width conversions.

For Intel there is a cpu detection thread safe fix for racing to init.

Full revision list
https://chromium.googlesource.com/libyuv/libyuv/+log/e6a54f22..ad36ba5c

R=xhwang@chromium.org
BUG=libyuv:510,libyuv:508,libyuv:511,libyuv:512

Review URL: https://codereview.chromium.org/1417343002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355673}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk