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This is a reland of https://codereview.chromium.org/578263003 using
buffer instead of memoryview, because on some early version of python
2.7, memoryview cannot be used with struct.unpack_from.

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

Review URL: https://codereview.chromium.org/588493002

Cr-Commit-Position: refs/heads/master@{#295684}