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This unblocks rolling WebRTC, since it depends on
libyuv and cannot use the dependency to ppapi, which
was removed in https://code.google.com/p/libyuv/source/detail?r=1130.

BUG=libyuv:374

Review URL: https://codereview.chromium.org/666103002

Cr-Commit-Position: refs/heads/master@{#300483}