Remove getting max payload length from default module.

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

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@8345 4adac7df-926f-26a2-2b94-8c16560cd09d
5 files changed