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

webrtc/modules/rtp_rtcp/source/rtcp_packet.h

Index: webrtc/modules/rtp_rtcp/source/rtcp_packet.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_packet.h b/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
index c5432de9d6030544f4d6ea91405950d1712cbce6..601e6366f132ea2daa6dc5aa4a260f03708afffd 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
@@ -94,16 +94,29 @@ class RtcpPacket {
  protected:
   RtcpPacket() {}
 
+  struct CommonHeader {
+    CommonHeader();
+
+    uint8_t count_or_format;
+    uint8_t payload_type;
+    uint32_t payload_size_bytes;
+  };
+
   virtual bool Create(uint8_t* packet,
                       size_t* index,
                       size_t max_length,
                       PacketReadyCallback* callback) const = 0;
 
-  void CreateHeader(uint8_t count_or_format,
-                    uint8_t packet_type,
-                    size_t length,
-                    uint8_t* buffer,
-                    size_t* pos) const;
+  static void CreateHeader(uint8_t count_or_format,
+                           uint8_t packet_type,
+                           size_t block_length,  // Size in 32bit words - 1.
+                           uint8_t* buffer,
+                           size_t* pos);
+
+  static bool ParseHeader(const uint8_t* packet,
+                          size_t index,
+                          size_t max_length,
+                          CommonHeader* header);
 
   bool OnBufferFull(uint8_t* packet,
                     size_t* index,