Require FilePathWatcher destructor to be called in sequence with Watch().

base/files/file_path_watcher.cc

 // 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.
 
 // Cross platform methods for FilePathWatcher. See the various platform
 // specific implementation files, too.
 
 #include "base/files/file_path_watcher.h"
 
 #include "base/logging.h"
-#include "base/message_loop/message_loop.h"
 #include "build/build_config.h"
 
 namespace base {
 
 FilePathWatcher::~FilePathWatcher() {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
   impl_->Cancel();
 }
 
 // static
 bool FilePathWatcher::RecursiveWatchAvailable() {
 #if (defined(OS_MACOSX) && !defined(OS_IOS)) || defined(OS_WIN) || \
     defined(OS_LINUX) || defined(OS_ANDROID)
   return true;
 #else
   // FSEvents isn't available on iOS.
   return false;
 #endif
 }
 
 FilePathWatcher::PlatformDelegate::PlatformDelegate(): cancelled_(false) {
 }
 
 FilePathWatcher::PlatformDelegate::~PlatformDelegate() {
   DCHECK(is_cancelled());
 }
 
 bool FilePathWatcher::Watch(const FilePath& path,
                             bool recursive,
                             const Callback& callback) {
+  DCHECK(sequence_checker_.CalledOnValidSequence());
   DCHECK(path.IsAbsolute());
   return impl_->Watch(path, recursive, callback);
 }
 
 }  // namespace base