Creating Simulcast offer and answer in Peer Connection.

CreateOffer and CreateAnswer will now examine the layers on the
transceiver to determine if multiple layers are requested (Simulcast).
In this scenario RIDs will be used in the layers (instead of SSRCs).
When the offer is created, only RIDs are signalled in the offer.
When the offer is set locally SetLocalDescription() SSRCs will be
generated for each layer by the Channel and sent downstream to the
MediaChannel.
The MediaChannel receives configuration that looks identical to that of
legacy simulcast, and should be able to integrate the streams correctly
regardless of how they were signalled.
Setting multiple layers on the transciever is still not supported
through the API.

Bug: webrtc:10075
Change-Id: Id4ad3637b87b68ef6ca7eec69166fee2d9dfa36f
Reviewed-on: https://webrtc-review.googlesource.com/c/119780
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Amit Hilbuch <amithi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26428}

pc/peer_connection.h

@@ -1149,6 +1149,12 @@ class PeerConnection : public PeerConnectionInternal,
 
   int usage_event_accumulator_ = 0;
   bool return_histogram_very_quickly_ = false;
+
+  // This object should be used to generate any SSRC that is not explicitly
+  // specified by the user (or by the remote party).
+  // The generator is not used directly, instead it is passed on to the
+  // channel manager and the session description factory.
+  rtc::UniqueRandomIdGenerator ssrc_generator_;
 };
 
 }  // namespace webrtc