Cleanup: Use uint8_t type throughout in the MD5 API.

Cleanup: Use uint8_t type throughout in the MD5 API.

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

Committed: https://crrev.com/6b950199c92a1fb530beb64110332b316528d9ae
Cr-Commit-Position: refs/heads/master@{#323949}

[tfarina, 2015-04-04 15:35:32]

PTAL!

[Lei Zhang, 2015-04-06 18:53:19]

lgtm

[tfarina, 2015-04-06 19:15:46]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-04-06 19:16:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1062673003/1

[commit-bot: I haz the power, 2015-04-06 21:04:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-06 21:06:01]

Patchset 1 (id:??) landed as
https://crrev.com/6b950199c92a1fb530beb64110332b316528d9ae
Cr-Commit-Position: refs/heads/master@{#323949}