Update TaskTraits in history_service.cc.

components/history/core/browser/history_service.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.
 
 // The history system runs on a background thread so that potentially slow
 // database operations don't delay the browser. This backend processing is
 // represented by HistoryBackend. The HistoryService's job is to dispatch to
 // that thread.
 //
 // Main thread                       History thread
@@ skipping 868 matching lines @@
     if (!thread_->StartWithOptions(options)) {
       Cleanup();
       return false;
     }
     backend_task_runner_ = thread_->task_runner();
   } else {
     backend_task_runner_ = base::CreateSequencedTaskRunnerWithTraits(
         base::TaskTraits()
             .WithPriority(base::TaskPriority::USER_BLOCKING)
             .WithShutdownBehavior(base::TaskShutdownBehavior::BLOCK_SHUTDOWN)
-            .WithFileIO());
+            .MayBlock());
   }
 
   // Create the history backend.
   scoped_refptr<HistoryBackend> backend(new HistoryBackend(
       new BackendDelegate(weak_ptr_factory_.GetWeakPtr(),
                           base::ThreadTaskRunnerHandle::Get()),
       history_client_ ? history_client_->CreateBackendClient() : nullptr,
       backend_task_runner_));
   history_backend_.swap(backend);
 
@@ skipping 248 matching lines @@
   return favicon_changed_callback_list_.Add(callback);
 }
 
 void HistoryService::NotifyFaviconsChanged(const std::set<GURL>& page_urls,
                                            const GURL& icon_url) {
   DCHECK(thread_checker_.CalledOnValidThread());
   favicon_changed_callback_list_.Notify(page_urls, icon_url);
 }
 
 }  // namespace history