Log stack trace when ObjectWatcher::StartWatching() is called.

base/win/object_watcher.h

 // Copyright (c) 2011 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.
 
 #ifndef BASE_WIN_OBJECT_WATCHER_H_
 #define BASE_WIN_OBJECT_WATCHER_H_
 #pragma once
 
 #include <windows.h>
 
 #include "base/base_export.h"
 #include "base/callback.h"
+#include "base/debug/stack_trace.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop.h"
 
 namespace base {
 namespace win {
 
 // A class that provides a means to asynchronously wait for a Windows object to
 // become signaled.  It is an abstraction around RegisterWaitForSingleObject
 // that provides a notification callback, OnObjectSignaled, that runs back on
 // the origin thread (i.e., the thread that called StartWatching).
@@ skipping 63 matching lines @@
   // MessageLoop::DestructionObserver implementation:
   virtual void WillDestroyCurrentMessageLoop();
 
   // Internal state.
   WeakPtrFactory<ObjectWatcher> weak_factory_;
   Closure callback_;
   HANDLE object_;             // The object being watched
   HANDLE wait_object_;        // Returned by RegisterWaitForSingleObject
   MessageLoop* origin_loop_;  // Used to get back to the origin thread
 
+  // For debugging crbug.com/121086.
+  base::debug::StackTrace stack_trace_;
+
   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 };
 
 }  // namespace win
 }  // namespace base
 
 #endif  // BASE_OBJECT_WATCHER_H_