DropPrivileges() is used in exception_port_tool, so that when it is
installed as a setuid executable, it only uses elevated privileges to
obtain a task port for its -p option, and then relinquishes those
privileges.
It is difficult to provide a test for this function, because it must be
running setuid or setgid in order to do anything interesting. However,
the function contains its own CHECKs to verify that it behaves properly.
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/727053002
