Add `quic::HandshakerDelegateInterface::OnTlsHandshakeConfirmed` and use it to increment the hyperloop's "handshake success" stats. PiperOrigin-RevId: 583041100

diff --git a/quiche/quic/core/handshaker_delegate_interface.h b/quiche/quic/core/handshaker_delegate_interface.h
index f10ba4a..a1c1a44 100644
--- a/quiche/quic/core/handshaker_delegate_interface.h
+++ b/quiche/quic/core/handshaker_delegate_interface.h

@@ -37,6 +37,10 @@
   // handshake.
   virtual void OnTlsHandshakeComplete() = 0;
 
+  // Called on the client side when handshake state change to
+  // HANDSHAKE_CONFIRMED. Only used in TLS handshake.
+  virtual void OnTlsHandshakeConfirmed() = 0;
+
   // Called to discard old decryption keys to stop processing packets of
   // encryption |level|.
   virtual void DiscardOldDecryptionKey(EncryptionLevel level) = 0;

diff --git a/quiche/quic/core/quic_session.h b/quiche/quic/core/quic_session.h
index dd757b7..db16a19 100644
--- a/quiche/quic/core/quic_session.h
+++ b/quiche/quic/core/quic_session.h

@@ -317,6 +317,7 @@
       EncryptionLevel level, std::unique_ptr<QuicEncrypter> encrypter) override;
   void SetDefaultEncryptionLevel(EncryptionLevel level) override;
   void OnTlsHandshakeComplete() override;
+  void OnTlsHandshakeConfirmed() override {}
   void DiscardOldDecryptionKey(EncryptionLevel level) override;
   void DiscardOldEncryptionKey(EncryptionLevel level) override;
   void NeuterUnencryptedData() override;

diff --git a/quiche/quic/core/tls_client_handshaker.cc b/quiche/quic/core/tls_client_handshaker.cc
index 0a9a7d2..244fbbb 100644
--- a/quiche/quic/core/tls_client_handshaker.cc
+++ b/quiche/quic/core/tls_client_handshaker.cc

@@ -478,6 +478,7 @@
     return;
   }
   state_ = HANDSHAKE_CONFIRMED;
+  handshaker_delegate()->OnTlsHandshakeConfirmed();
   handshaker_delegate()->DiscardOldEncryptionKey(ENCRYPTION_HANDSHAKE);
   handshaker_delegate()->DiscardOldDecryptionKey(ENCRYPTION_HANDSHAKE);
 }