Fix a type cast issue for compiling webrtc  with BoringSSL.

BUG=
R=juberti@google.com, juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16039004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6672 4adac7df-926f-26a2-2b94-8c16560cd09d