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

base/files/file_descriptor_watcher_posix.cc

 // Copyright 2016 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 "base/files/file_descriptor_watcher_posix.h"
 
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 74 matching lines @@
   // MessageLoopForIO thread.
   bool registered_as_destruction_observer_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(Watcher);
 };
 
 FileDescriptorWatcher::Controller::Watcher::Watcher(
     WeakPtr<Controller> controller,
     MessageLoopForIO::Mode mode,
     int fd)
-    : controller_(controller), mode_(mode), fd_(fd) {
+    : file_descriptor_watcher_(FROM_HERE),
+      controller_(controller),
+      mode_(mode),
+      fd_(fd) {
   DCHECK(callback_task_runner_);
   thread_checker_.DetachFromThread();
 }
 
 FileDescriptorWatcher::Controller::Watcher::~Watcher() {
   DCHECK(thread_checker_.CalledOnValidThread());
   MessageLoopForIO::current()->RemoveDestructionObserver(this);
 }
 
 void FileDescriptorWatcher::Controller::Watcher::StartWatching() {
@@ skipping 92 matching lines @@
   return WrapUnique(new Controller(MessageLoopForIO::WATCH_READ, fd, callback));
 }
 
 std::unique_ptr<FileDescriptorWatcher::Controller>
 FileDescriptorWatcher::WatchWritable(int fd, const Closure& callback) {
   return WrapUnique(
       new Controller(MessageLoopForIO::WATCH_WRITE, fd, callback));
 }
 
 }  // namespace base