Introduce helpers to RtpSender to propagate RtpPacketToSend.

webrtc/modules/rtp_rtcp/source/rtp_sender.h

Index: webrtc/modules/rtp_rtcp/source/rtp_sender.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender.h b/webrtc/modules/rtp_rtcp/source/rtp_sender.h
index 7a6677b408e29b29a716d996b761d4059d0607d5..6c9928c79312b98ce505d0b16b05c98adb10d397 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender.h
@@ -203,6 +203,14 @@ class RTPSender {
 
   void SetRtxPayloadType(int payload_type, int associated_payload_type);
 
+  // Create empty packet, fills ssrc, csrcs and reserve place for header
+  // extensions RtpSender updates before sending.
+  std::unique_ptr<RtpPacketToSend> AllocatePacket() const;
+  // Allocate sequence number for provided packet.
+  // Save packet's fields to generate padding that doesn't break media stream.
+  // Return false if sending was turned off.
+  bool AssignSequenceNumber(RtpPacketToSend* packet);
+
   // Functions wrapping RTPSenderInterface.
   int32_t BuildRTPheader(uint8_t* data_buffer,
                          int8_t payload_type,