DCHECK: Reference condition parameter in release builds

So that caller's won't get warnings about unused variables for
variables that are only used in calls to DCHECK, such as

  int x = ...
  DCHECK_EQ(x, 17);

R=andrew@webrtc.org

Previously committed: https://code.google.com/p/webrtc/source/detail?r=7858
and reverted: https://code.google.com/p/webrtc/source/detail?r=7859

Review URL: https://webrtc-codereview.appspot.com/31169004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7869 4adac7df-926f-26a2-2b94-8c16560cd09d