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This puts the declaration of MD5Sum() function in sync with the
definition, which is right after MD5DigestToBase16() in the source file,
so it should be in the header file as well.

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

Review URL: https://codereview.chromium.org/1061903004

Cr-Commit-Position: refs/heads/master@{#324058}