Redirect HistoryService thread to TaskScheduler via a field trial.

components/history/core/browser/history_service.h

 // 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.
 
 #ifndef COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_SERVICE_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_SERVICE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <set>
 #include <string>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/callback_list.h"
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
+#include "base/sequenced_task_runner.h"
 #include "base/strings/string16.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "base/threading/thread_checker.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
 #include "components/favicon_base/favicon_callback.h"
 #include "components/favicon_base/favicon_usage_data.h"
 #include "components/history/core/browser/delete_directive_handler.h"
 #include "components/history/core/browser/history_types.h"
 #include "components/history/core/browser/keyword_id.h"
@@ skipping 753 matching lines @@
   // Called when the favicons for the given page URLs (e.g.
   // http://www.google.com) and the given icon URL (e.g.
   // http://www.google.com/favicon.ico) have changed. It is valid to call
   // NotifyFaviconsChanged() with non-empty |page_urls| and an empty |icon_url|
   // and vice versa.
   void NotifyFaviconsChanged(const std::set<GURL>& page_urls,
                              const GURL& icon_url);
 
   base::ThreadChecker thread_checker_;
 
-  // The thread used by the history service to run complicated operations.
-  // |thread_| is null once Cleanup() is called.
-  base::Thread* thread_;
+  // The thread used by the history service to run HistoryBackend operations.
+  // Intentionally not a BrowserThread because the sync integration unit tests
+  // need to create multiple HistoryServices which each have their own thread.
+  // Nullptr if TaskScheduler is used for HistoryBackend operations.
+  std::unique_ptr<base::Thread> thread_;
+
+  // The TaskRunner to which HistoryBackend tasks are posted. Nullptr once
+  // Cleanup() is called.
+  scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
 
   // This class has most of the implementation and runs on the 'thread_'.
   // You MUST communicate with this class ONLY through the thread_'s
   // message_loop().
   //
   // This pointer will be null once Cleanup() has been called, meaning no
   // more calls should be made to the history thread.
   scoped_refptr<HistoryBackend> history_backend_;
 
   // A cache of the user-typed URLs kept in memory that is used by the
@@ skipping 21 matching lines @@
 
   // All vended weak pointers are invalidated in Cleanup().
   base::WeakPtrFactory<HistoryService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryService);
 };
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_SERVICE_H_