Add stereo codec header and pass it through RTP

- Defines CodecSpecificInfoStereo that carries stereo specific header info from
encoded image.
- Defines RTPVideoHeaderStereo that carries the above info to packetizer,
see module_common_types.h.
- Adds an RTPPacketizer and RTPDepacketizer that supports passing specific stereo
header.
- Uses new data containers in StereoAdapter classes.

This CL is the step 3 for adding alpha channel support over the wire in webrtc.
See https://webrtc-review.googlesource.com/c/src/+/7800 for the experimental
CL that gives an idea about how it will come together.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: Ia932568fdd7065ba104afd2bc0ecf25a765748ab
Reviewed-on: https://webrtc-review.googlesource.com/22900
Reviewed-by: Emircan Uysaler <emircan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20920}

modules/video_coding/frame_object.cc

@@ -43,9 +43,14 @@ RtpFrameObject::RtpFrameObject(PacketBuffer* packet_buffer,
   frame_type_ = first_packet->frameType;
   codec_type_ = first_packet->codec;
 
+  // Stereo codec appends CopyCodecSpecific to last packet to avoid copy.
+  VCMPacket* packet_with_codec_specific =
+      codec_type_ == kVideoCodecStereo ? packet_buffer_->GetPacket(last_seq_num)
+                                       : first_packet;
+
   // TODO(philipel): Remove when encoded image is replaced by FrameObject.
   // VCMEncodedFrame members
-  CopyCodecSpecific(&first_packet->video_header);
+  CopyCodecSpecific(&packet_with_codec_specific->video_header);
   _completeFrame = true;
   _payloadType = first_packet->payloadType;
   _timeStamp = first_packet->timestamp;