Fixed shutdown concurrency issues in FilePathWatcher.

chrome/browser/file_path_watcher/file_path_watcher.cc

 // Copyright (c) 2010 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.
 
 // Cross platform methods for FilePathWatcher. See the various platform
 // specific implementation files, too.
 
 #include "chrome/browser/file_path_watcher/file_path_watcher.h"
 
+#include "content/browser/browser_thread.h"
+
 FilePathWatcher::~FilePathWatcher() {
   impl_->Cancel();
 }
 
 bool FilePathWatcher::Watch(const FilePath& path,
                             Delegate* delegate) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   DCHECK(path.IsAbsolute());
   return impl_->Watch(path, delegate);
 }