Add a ParseHeader method to RtcpPacket, for parsing common RTCP header.

webrtc/modules/rtp_rtcp/source/rtcp_packet.cc

Index: webrtc/modules/rtp_rtcp/source/rtcp_packet.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_packet.cc b/webrtc/modules/rtp_rtcp/source/rtcp_packet.cc
index 8cb6fb0833644077c3aed6f60475b56e9f23b2cc..e5ea37e001ea130ab18436b78c4b52871b902457 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_packet.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_packet.cc
@@ -743,7 +743,7 @@ void RtcpPacket::CreateHeader(
     uint8_t packet_type,
     size_t length,
     uint8_t* buffer,
-    size_t* pos) const {
+    size_t* pos) {
   assert(length <= 0xffff);
   const uint8_t kVersion = 2;
   AssignUWord8(buffer, pos, (kVersion << 6) + count_or_format);