Add virtual method rtcp::RtcpPacket::SetSenderSsrc

This will allow RtcpPackets to be sent in a more generic way where the
PacketRouter does not have to know about the type.

App::SetSsrc is replaced with SetSenderSsrc

Bug: webrtc:10742
Change-Id: I9fa18d408250f15818dc6898093d9b116603facb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156166
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29420}

modules/rtp_rtcp/source/rtcp_packet/app.h

@@ -31,7 +31,7 @@ class App : public RtcpPacket {
   // Parse assumes header is already parsed and validated.
   bool Parse(const CommonHeader& packet);
 
-  void SetSsrc(uint32_t ssrc) { ssrc_ = ssrc; }
+  void SetSenderSsrc(uint32_t ssrc) override { ssrc_ = ssrc; }
   void SetSubType(uint8_t subtype);
   void SetName(uint32_t name) { name_ = name; }
   void SetData(const uint8_t* data, size_t data_length);