Call WatcherManager functions on the IO thread.

chrome/browser/chromeos/file_system_provider/fileapi/backend_delegate.cc

 // Copyright 2014 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/file_system_provider/fileapi/backend_delegate.h"
 
 #include <memory>
 
 #include "chrome/browser/chromeos/file_system_provider/fileapi/buffering_file_stream_reader.h"
 #include "chrome/browser/chromeos/file_system_provider/fileapi/buffering_file_stream_writer.h"
@@ skipping 61 matching lines @@
   DCHECK_EQ(storage::kFileSystemTypeProvided, url.type());
 
   return std::unique_ptr<storage::FileStreamWriter>(
       new BufferingFileStreamWriter(std::unique_ptr<storage::FileStreamWriter>(
                                         new FileStreamWriter(url, offset)),
                                     kWriterBufferSize));
 }
 
 storage::WatcherManager* BackendDelegate::GetWatcherManager(
     storage::FileSystemType type) {
-  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK_EQ(storage::kFileSystemTypeProvided, type);
   return watcher_manager_.get();
 }
 
 void BackendDelegate::GetRedirectURLForContents(
     const storage::FileSystemURL& url,
     const storage::URLCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK_EQ(storage::kFileSystemTypeProvided, url.type());
   callback.Run(GURL());
 }
 
 }  // namespace file_system_provider
 }  // namespace chromeos