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This is a reland of https://codereview.chromium.org/588493002 using buffer on
bytearray before trying to call unpack_from because python 2.7.3 and earlier
doesn't handle unpacking a bytearray directly.

BUG=415491
R=sdefresne@chromium.org,cmasone@chromium.org

Review URL: https://codereview.chromium.org/584893002

Cr-Commit-Position: refs/heads/master@{#295716}