Add UMA for the kernel's TCP RTT estimate.

net/socket/tcp_socket_libevent.h

 // Copyright 2013 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.
 
 #ifndef NET_SOCKET_TCP_SOCKET_LIBEVENT_H_
 #define NET_SOCKET_TCP_SOCKET_LIBEVENT_H_
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
 #include "net/base/address_family.h"
 #include "net/base/completion_callback.h"
 #include "net/base/net_export.h"
 #include "net/log/net_log.h"
 
 namespace net {
 
 class AddressList;
 class IOBuffer;
 class IPEndPoint;
@@ skipping 34 matching lines @@
   // The commonly used options for client sockets and accepted sockets:
   // - SetNoDelay(true);
   // - SetKeepAlive(true, 45).
   void SetDefaultOptionsForClient();
   int SetAddressReuse(bool allow);
   int SetReceiveBufferSize(int32 size);
   int SetSendBufferSize(int32 size);
   bool SetKeepAlive(bool enable, int delay);
   bool SetNoDelay(bool no_delay);
 
+  // Gets the estimated RTT. Returns false if the RTT is
+  // unavailable. May also return false when estimated RTT is 0.
+  bool GetEstimatedRoundTripTime(base::TimeDelta* out_rtt) const
+      WARN_UNUSED_RESULT;
+
   void Close();
 
   // Setter/Getter methods for TCP FastOpen socket option.
   bool UsingTCPFastOpen() const;
   void EnableTCPFastOpenIfSupported();
 
   bool IsValid() const;
 
   // Marks the start/end of a series of connect attempts for logging purpose.
   //
@@ skipping 129 matching lines @@
   bool logging_multiple_connect_attempts_;
 
   BoundNetLog net_log_;
 
   DISALLOW_COPY_AND_ASSIGN(TCPSocketLibevent);
 };
 
 }  // namespace net
 
 #endif  // NET_SOCKET_TCP_SOCKET_LIBEVENT_H_