Add UMA for the kernel's TCP RTT estimate.

net/socket/tcp_socket_libevent.h

Index: net/socket/tcp_socket_libevent.h
diff --git a/net/socket/tcp_socket_libevent.h b/net/socket/tcp_socket_libevent.h
index 78b035d2d8d892385aeb8037b30b3c41ea437726..9f895e2696ae47da79aefedcca1de0ab7b33066b 100644
--- a/net/socket/tcp_socket_libevent.h
+++ b/net/socket/tcp_socket_libevent.h
@@ -9,6 +9,7 @@
 #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"
@@ -63,6 +64,9 @@ class NET_EXPORT TCPSocketLibevent {
   bool SetKeepAlive(bool enable, int delay);
   bool SetNoDelay(bool no_delay);
 
+  // Gets the estimated RTT. Returns false if the RTT is unavailable.
+  bool GetEstimatedRoundTripTime(base::TimeDelta* out_rtt);
+
   void Close();
 
   // Setter/Getter methods for TCP FastOpen socket option.