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

Also refactor TransportFeedback to use this.

BUG=

Review URL: https://codereview.webrtc.org/1307663004

Cr-Commit-Position: refs/heads/master@{#9935}

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);