Fix C++11 -Wnarrowing in channel_unittest.cc.

Implicit conversion from int to unsigned char inside {} initializers is
ill-formed C++11 and triggers a warning in clang when building it as
such.

BUG=
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12709004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6351 4adac7df-926f-26a2-2b94-8c16560cd09d