Allow using system libevent instead of the bundled one.

net/socket/tcp_client_socket_libevent.cc

 // Copyright (c) 2006-2008 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.
 
 #include "net/socket/tcp_client_socket_libevent.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <netdb.h>
 #include <sys/socket.h>
 
 #include "base/eintr_wrapper.h"
 #include "base/message_loop.h"
 #include "base/string_util.h"
 #include "base/trace_event.h"
 #include "net/base/io_buffer.h"
 #include "net/base/load_log.h"
 #include "net/base/net_errors.h"
+#if defined(USE_SYSTEM_LIBEVENT)
+#include <event.h>
+#else
 #include "third_party/libevent/event.h"
-
+#endif
 
 namespace net {
 
 namespace {
 
 const int kInvalidSocket = -1;
 
 // Return 0 on success, -1 on failure.
 // Too small a function to bother putting in a library?
 int SetNonBlocking(int fd) {
@@ skipping 415 matching lines @@
     DoWriteCallback(result);
   }
 }
 
 int TCPClientSocketLibevent::GetPeerName(struct sockaddr* name,
                                          socklen_t* namelen) {
   return ::getpeername(socket_, name, namelen);
 }
 
 }  // namespace net