The entire code base prefers c++ style casting, but there is
the only occurrence of C++ cast (a static_cast), the rest
of this file is using C-cast style.

So we go the other way round and converted everything from
C to C++ way of casting.

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

Review URL: https://codereview.chromium.org/1044423007

Cr-Commit-Position: refs/heads/master@{#326795}