Call WatcherManager functions on the IO thread.

chrome/browser/chromeos/fileapi/mtp_watcher_manager.cc

 // Copyright 2015 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 "chrome/browser/chromeos/fileapi/mtp_watcher_manager.h"
 
+#include "content/public/browser/browser_thread.h"
+
+using content::BrowserThread;
+
 namespace chromeos {
 
 MTPWatcherManager::MTPWatcherManager(
     DeviceMediaAsyncFileUtil* device_media_async_file_util)
     : device_media_async_file_util_(device_media_async_file_util) {
   DCHECK(device_media_async_file_util != NULL);
 }
 
 MTPWatcherManager::~MTPWatcherManager() {
 }
 
 void MTPWatcherManager::AddWatcher(
     const storage::FileSystemURL& url,
     bool recursive,
     const StatusCallback& callback,
     const NotificationCallback& notification_callback) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
   device_media_async_file_util_->AddWatcher(url, recursive, callback,
                                             notification_callback);
 }
 
 void MTPWatcherManager::RemoveWatcher(const storage::FileSystemURL& url,
                                       bool recursive,
                                       const StatusCallback& callback) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
   device_media_async_file_util_->RemoveWatcher(url, recursive, callback);
 }
 
 }  // namespace chromeos