Refactoring: Create per-connection packet writers in QuicDispatcher.

net/quic/quic_server_session.cc

Index: net/quic/quic_server_session.cc
diff --git a/net/quic/quic_server_session.cc b/net/quic/quic_server_session.cc
index 399b772e578bc18508713e26bc6298ec6de94a3c..c7141e6568fb34e71e2ef8759b399bfd859833d5 100644
--- a/net/quic/quic_server_session.cc
+++ b/net/quic/quic_server_session.cc
@@ -15,10 +15,8 @@ namespace net {
 QuicServerSession::QuicServerSession(
     const QuicConfig& config,
     QuicConnection* connection,
-    QuicPerConnectionPacketWriter* connection_packet_writer,
     QuicServerSessionVisitor* visitor)
     : QuicSession(connection, config),
-      connection_packet_writer_(connection_packet_writer),
       visitor_(visitor) {}
 
 QuicServerSession::~QuicServerSession() {}