Use the location where FileDescriptorWatcher is created to track the libevent handlers

net/dns/notify_watcher_mac.cc

 // 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.
 
 #include "net/dns/notify_watcher_mac.h"
 
 #include <notify.h>
 
 #include "base/logging.h"
 #include "base/posix/eintr_wrapper.h"
 
 namespace net {
 
-NotifyWatcherMac::NotifyWatcherMac() : notify_fd_(-1), notify_token_(-1) {}
+NotifyWatcherMac::NotifyWatcherMac()
+    : notify_fd_(-1), notify_token_(-1), watcher_(FROM_HERE) {}
 
 NotifyWatcherMac::~NotifyWatcherMac() {
   Cancel();
 }
 
 bool NotifyWatcherMac::Watch(const char* key, const CallbackType& callback) {
   DCHECK(key);
   DCHECK(!callback.is_null());
   Cancel();
   uint32_t status = notify_register_file_descriptor(
@@ skipping 30 matching lines @@
     Cancel();
     callback_.Run(false);
     return;
   }
   // Ignoring |token| value to avoid possible endianness mismatch:
   // http://openradar.appspot.com/8821081
   callback_.Run(true);
 }
 
 }  // namespace net