Correcting the amount of padding when send side bwe includes RTP overhead.

webrtc/modules/rtp_rtcp/source/rtp_packet.h

Index: webrtc/modules/rtp_rtcp/source/rtp_packet.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_packet.h b/webrtc/modules/rtp_rtcp/source/rtp_packet.h
index 3c02125e082bfbce2921bd6057f9f4bcc74d9bdc..578ea2cd03bf241f570a9bfbd972dbae7d0a12c7 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_packet.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_packet.h
@@ -51,7 +51,7 @@ class Packet {
   // directly. Function is there just for easier backward compatibilty.
   void GetHeader(RTPHeader* header) const;
 
-  size_t headers_size() const;
+  virtual size_t headers_size() const;
 
   // Payload.
   size_t payload_size() const;
@@ -61,7 +61,7 @@ class Packet {
   // Buffer.
   rtc::CopyOnWriteBuffer Buffer() const;
   size_t capacity() const;
-  size_t size() const;
+  virtual size_t size() const;
   const uint8_t* data() const;
   size_t FreeCapacity() const;
   size_t MaxPayloadSize() const;