Cleanup: Match the declaration order of MD5Sum() with the definition.

Cleanup: Match the declaration order of MD5Sum() with the definition.

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

Committed: https://crrev.com/709579eedbd4c8a5d116398bd8b76e7c13366c22
Cr-Commit-Position: refs/heads/master@{#324058}

[Lei Zhang, 2015-04-06 21:54:55]

lgtm

[tfarina, 2015-04-07 14:04:09]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-04-07 14:04:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1061903004/1

[commit-bot: I haz the power, 2015-04-07 16:57:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-07 17:01:25]

Patchset 1 (id:??) landed as
https://crrev.com/709579eedbd4c8a5d116398bd8b76e7c13366c22
Cr-Commit-Position: refs/heads/master@{#324058}