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This seems to be more consistent with the use of uint32_t type that is
also used throughout in this API.

BUG=None
TEST=base_unittests --gtest_filter=MD5*
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1062673003

Cr-Commit-Position: refs/heads/master@{#323949}