Adds plumbing for QUIC early connection migration. No behavior change.

net/quic/quic_connection.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // The entity that handles framing writes for a Quic client or server.
 // Each QuicSession will have a connection associated with it.
 //
 // On the server side, the Dispatcher handles the raw reads, and hands off
 // packets via ProcessUdpPacket for framing and processing.
 //
@@ skipping 114 matching lines @@
 
   // Called when a blocked socket becomes writable.
   virtual void OnCanWrite() = 0;
 
   // Called when the connection experiences a change in congestion window.
   virtual void OnCongestionWindowChange(QuicTime now) = 0;
 
   // Called when the connection receives a packet from a migrated client.
   virtual void OnConnectionMigration() = 0;
 
+  // Called when the peer seems unreachable over the current path.
+  virtual void OnPathDegrading() = 0;
+
   // Called after OnStreamFrame, OnRstStream, OnGoAway, OnWindowUpdateFrame,
   // OnBlockedFrame, and OnCanWrite to allow post-processing once the work has
   // been done.
   virtual void PostProcessAfterData() = 0;
 
   // Called to ask if the visitor wants to schedule write resumption as it both
   // has pending data to write, and is able to write (e.g. based on flow control
   // limits).
   // Writes may be pending because they were write-blocked, congestion-throttled
   // or yielded to other connections.
@@ skipping 313 matching lines @@
   void PopulateAckFrame(QuicAckFrame* ack) override;
   void PopulateStopWaitingFrame(QuicStopWaitingFrame* stop_waiting) override;
 
   // QuicPacketCreator::DelegateInterface
   void OnSerializedPacket(SerializedPacket* packet) override;
   void OnResetFecGroup() override;
 
   // QuicSentPacketManager::NetworkChangeVisitor
   void OnCongestionWindowChange() override;
   void OnRttChange() override;
+  void OnPathDegrading() override;
 
   // Called by the crypto stream when the handshake completes. In the server's
   // case this is when the SHLO has been ACKed. Clients call this on receipt of
   // the SHLO.
   void OnHandshakeComplete();
 
   // Accessors
   void set_visitor(QuicConnectionVisitorInterface* visitor) {
     visitor_ = visitor;
   }
@@ skipping 560 matching lines @@
 
   // If true, multipath is enabled for this connection.
   bool multipath_enabled_;
 
   DISALLOW_COPY_AND_ASSIGN(QuicConnection);
 };
 
 }  // namespace net
 
 #endif  // NET_QUIC_QUIC_CONNECTION_H_