Use RtpPacketToSend in RtpSenderVideo

webrtc/modules/rtp_rtcp/source/rtp_packet.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_packet.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_packet.cc b/webrtc/modules/rtp_rtcp/source/rtp_packet.cc
index 283512cd8fa51e7d7dc74da62f5cc116f9051ad3..2362ff23934b0ac3e623a28bafa8219d688d1df8 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_packet.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_packet.cc
@@ -274,6 +274,9 @@ uint8_t* Packet::AllocatePayload(size_t size_bytes) {
     LOG(LS_WARNING) << "Cannot set payload, not enough space in buffer.";
     return nullptr;
   }
+  // Reset payload size to 0. If CopyOnWrite buffer_ was shared, this will cause
+  // reallocation and memcpy. Setting size to just headers reduces memcpy size.
+  buffer_.SetSize(payload_offset_);
   payload_size_ = size_bytes;
   buffer_.SetSize(payload_offset_ + payload_size_);
   return WriteAt(payload_offset_);