Allow using system libevent instead of the bundled one.

net/base/listen_socket.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 "build/build_config.h"
 
 #if defined(OS_WIN)
 // winsock2.h must be included first in order to ensure it is included before
 // windows.h.
 #include <winsock2.h>
 #elif defined(OS_POSIX)
 #include <errno.h>
 #include <sys/socket.h>
 #include <arpa/inet.h>
 #include "net/base/net_errors.h"
+#if defined(USE_SYSTEM_LIBEVENT)
+#include <event.h>
+#else
 #include "third_party/libevent/event.h"
 #endif
+#endif
 
 #include "base/eintr_wrapper.h"
 #include "net/base/net_util.h"
 #include "net/base/listen_socket.h"
 
 #if defined(OS_WIN)
 typedef int socklen_t;
 #endif  // defined(OS_WIN)
 
 namespace {
@@ skipping 264 matching lines @@
   }
 }
 
 void ListenSocket::OnFileCanWriteWithoutBlocking(int fd) {
   // MessagePumpLibevent callback, we don't listen for write events
   // so we shouldn't ever reach here.
   NOTREACHED();
 }
 
 #endif