Roll ffmpeg DEPS for security fix.

      Pulls in:
525a71a avcodec/vp8: Do not use num_coeff_partitions in thread/buffer setup

BUG=532967
TEST=ffmpeg_regression_tests
TBR=chcunningham

Review URL: https://codereview.chromium.org/1376913003

Cr-Commit-Position: refs/heads/master@{#351889}