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/channel_interface.h

@@ -12,6 +12,7 @@
 #define PC_CHANNEL_INTERFACE_H_
 
 #include <string>
+#include <vector>
 
 #include "api/jsep.h"
 #include "api/media_types.h"
@@ -52,6 +53,10 @@ class ChannelInterface {
                                 webrtc::SdpType type,
                                 std::string* error_desc) = 0;
 
+  // Access to the local and remote streams that were set on the channel.
+  virtual const std::vector<StreamParams>& local_streams() const = 0;
+  virtual const std::vector<StreamParams>& remote_streams() const = 0;
+
   // Set an RTP level transport.
   // Some examples:
   //   * An RtpTransport without encryption.