Remove FilePathWatcher::PlatformDelegate::CancelOnMessageLoopThread().

base/files/file_path_watcher_stub.cc

 // Copyright (c) 2012 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.
 
 // This file exists for Unix systems which don't have the inotify headers, and
 // thus cannot build file_watcher_inotify.cc
 
 #include "base/files/file_path_watcher.h"
 
 namespace base {
 
 namespace {
 
 class FilePathWatcherImpl : public FilePathWatcher::PlatformDelegate {
  public:
   bool Watch(const FilePath& path,
              bool recursive,
              const FilePathWatcher::Callback& callback) override {
     return false;
   }
 
   void Cancel() override {}
 
-  void CancelOnMessageLoopThread() override {}
-
  protected:
   ~FilePathWatcherImpl() override {}
 };
 
 }  // namespace
 
 FilePathWatcher::FilePathWatcher() {
   impl_ = new FilePathWatcherImpl();
 }
 
 }  // namespace base