Extract RTP-header SSRC inline in Call.

Prevents unknown-RTP-header-extension warnings to be flooding from the
RTP-header parsing as there's no way to register RTP extensions for the
parser in Call as they're allowed to differ between RTP streams.

RTP-header parsing should instead be done separately in every
VideoReceiveStream.

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16829004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6619 4adac7df-926f-26a2-2b94-8c16560cd09d