Call WatcherManager functions on the IO thread.

chrome/browser/chromeos/arc/fileapi/arc_documents_provider_backend_delegate.cc

 // Copyright 2016 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/arc/fileapi/arc_documents_provider_backend_delegate.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 38 matching lines @@
     const storage::FileSystemURL& url,
     int64_t offset,
     storage::FileSystemContext* context) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   NOTREACHED();  // Read-only file system.
   return nullptr;
 }
 
 storage::WatcherManager* ArcDocumentsProviderBackendDelegate::GetWatcherManager(
     storage::FileSystemType type) {
-  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
   NOTREACHED();  // Non-watchable file system.
   return nullptr;
 }
 
 void ArcDocumentsProviderBackendDelegate::GetRedirectURLForContents(
     const storage::FileSystemURL& url,
     const storage::URLCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   NOTREACHED();  // Never called by chromeos::FileSystemBackend.
   callback.Run(GURL());
 }
 
 }  // namespace arc