This allows FFmpegVideoDecoder to log messages for web developers.

As a first attempt, this CL logs a message when FFmpeg fails to
decode a frame that includes the message from FFmpeg and details
about the failed buffer.

Includes FFmpeg DEPS roll to expose these messages.

    Roll src/third_party/ffmpeg/ 6d2e44445..9751ef06f (1 commit)

    https://chromium.googlesource.com/chromium/third_party/ffmpeg.git/+log/6d2e444456b7..9751ef06f90f

    $ git log 6d2e44445..9751ef06f --date=short --no-merges --format='%ad %ae %s'
    2017-03-23 sandersd Expose av_strerror() via ffmpeg.sigs.

BUG=TBD
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2768713002
Cr-Commit-Position: refs/heads/master@{#459548}