Introduce helpers to RtpSender to propagate RtpPacketToSend.

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 f1e3dd40f90cebcf80e3abe2cd4674f06249122c..6981eabc254f09d5314fbf4f875377d38f04ea4c 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_packet.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_packet.h
@@ -83,6 +83,9 @@ class Packet {
   void SetCsrcs(const std::vector<uint32_t>& csrcs);
 
   // Header extensions.
+  template <typename Extension>
+  bool HasExtension() const;
+
   template <typename Extension, typename... Values>
   bool GetExtension(Values...) const;
 
@@ -160,6 +163,12 @@ class Packet {
   Packet() = delete;
 };
 
+template <typename Extension>
+bool Packet::HasExtension() const {
+  uint16_t offset = 0;
+  return FindExtension(Extension::kId, Extension::kValueSizeBytes, &offset);
+}
+
 template <typename Extension, typename... Values>
 bool Packet::GetExtension(Values... values) const {
   uint16_t offset = 0;