If SRTP sessions exist, don't create new ones when applying answer.

pc/srtptransport.cc

Index: pc/srtptransport.cc
diff --git a/pc/srtptransport.cc b/pc/srtptransport.cc
index f3b5309e37c0c56faefb81740b96cfcde6dae61e..64889c65f69be77c77049615d76a3c73204953e8 100644
--- a/pc/srtptransport.cc
+++ b/pc/srtptransport.cc
@@ -174,25 +174,42 @@ bool SrtpTransport::SetRtpParams(int send_cs,
                                  int recv_cs,
                                  const uint8_t* recv_key,
                                  int recv_key_len) {
-  CreateSrtpSessions();
+  // If parameters are being set for the first time, we should create new SRTP
+  // sessions and call "SetSend/SetRecv". Otherwise we should call
+  // "UpdateSend"/"UpdateRecv" on the existing sessions, which will internally
+  // call "srtp_update".
+  bool new_sessions = false;
+  if (!send_session_) {
+    RTC_DCHECK(!recv_session_);
+    CreateSrtpSessions();
+    new_sessions = true;
+  }
+
   send_session_->SetEncryptedHeaderExtensionIds(
       send_encrypted_header_extension_ids_);
   if (external_auth_enabled_) {
     send_session_->EnableExternalAuth();
   }
-  if (!send_session_->SetSend(send_cs, send_key, send_key_len)) {
+  bool ret = new_sessions
+                 ? send_session_->SetSend(send_cs, send_key, send_key_len)
+                 : send_session_->UpdateSend(send_cs, send_key, send_key_len);
+  if (!ret) {
     ResetParams();
     return false;
   }
 
   recv_session_->SetEncryptedHeaderExtensionIds(
       recv_encrypted_header_extension_ids_);
-  if (!recv_session_->SetRecv(recv_cs, recv_key, recv_key_len)) {
+  ret = new_sessions
+            ? recv_session_->SetRecv(recv_cs, recv_key, recv_key_len)
+            : recv_session_->UpdateRecv(recv_cs, recv_key, recv_key_len);
+  if (!ret) {
     ResetParams();
     return false;
   }
 
-  LOG(LS_INFO) << "SRTP activated with negotiated parameters:"
+  LOG(LS_INFO) << "SRTP " << (new_sessions ? "updated" : "activated")
+               << " with negotiated parameters:"
                << " send cipher_suite " << send_cs << " recv cipher_suite "
                << recv_cs;
   return true;