Moving functionality to get max payload length from default RTP module to the payload router. I'll make a follow up CL changing asserts to DCHECK in rtp_rtcp_impl.cc. BUG=769 TEST=New unittest and existing sender mtu test R=stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/36119004 Cr-Commit-Position: refs/heads/master@{#8345} git-svn-id: http://webrtc.googlecode.com/svn/trunk@8345 4adac7df-926f-26a2-2b94-8c16560cd09d