Change SdpSerializeCandidate to output candidate line without the "a=" and without the leading \r\n", i.e. candidate-attribute as defined in section 15.1 of [ICE].

BUG=crbug/387632
R=juberti@google.com

Review URL: https://webrtc-codereview.appspot.com/17779004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6533 4adac7df-926f-26a2-2b94-8c16560cd09d