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

mojo/edk/system/channel_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 "mojo/edk/system/channel.h"
 
 #include <errno.h>
 #include <sys/socket.h>
 
 #include <algorithm>
@@ skipping 192 matching lines @@
   ~ChannelPosix() override {
     DCHECK(!read_watcher_);
     DCHECK(!write_watcher_);
     for (auto handle : incoming_platform_handles_)
       handle.CloseIfNecessary();
   }
 
   void StartOnIOThread() {
     DCHECK(!read_watcher_);
     DCHECK(!write_watcher_);
-    read_watcher_.reset(new base::MessageLoopForIO::FileDescriptorWatcher);
+    read_watcher_.reset(
+        new base::MessageLoopForIO::FileDescriptorWatcher(FROM_HERE));
     base::MessageLoop::current()->AddDestructionObserver(this);
     if (handle_.get().needs_connection) {
       base::MessageLoopForIO::current()->WatchFileDescriptor(
           handle_.get().handle, false /* persistent */,
           base::MessageLoopForIO::WATCH_READ, read_watcher_.get(), this);
     } else {
-      write_watcher_.reset(new base::MessageLoopForIO::FileDescriptorWatcher);
+      write_watcher_.reset(
+          new base::MessageLoopForIO::FileDescriptorWatcher(FROM_HERE));
       base::MessageLoopForIO::current()->WatchFileDescriptor(
           handle_.get().handle, true /* persistent */,
           base::MessageLoopForIO::WATCH_READ, read_watcher_.get(), this);
       base::AutoLock lock(write_lock_);
       FlushOutgoingMessagesNoLock();
     }
   }
 
   void WaitForWriteOnIOThread() {
     base::AutoLock lock(write_lock_);
@@ skipping 328 matching lines @@
 // static
 scoped_refptr<Channel> Channel::Create(
     Delegate* delegate,
     ScopedPlatformHandle platform_handle,
     scoped_refptr<base::TaskRunner> io_task_runner) {
   return new ChannelPosix(delegate, std::move(platform_handle), io_task_runner);
 }
 
 }  // namespace edk
 }  // namespace mojo