[reland] Add a default task runner argument to the constructor of JsonPrefStore.

components/prefs/json_pref_store.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_PREFS_JSON_PREF_STORE_H_
 #define COMPONENTS_PREFS_JSON_PREF_STORE_H_
 
 #include <stdint.h>
 
 #include <memory>
 #include <set>
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/files/important_file_writer.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/non_thread_safe.h"
 #include "components/prefs/base_prefs_export.h"
 #include "components/prefs/persistent_pref_store.h"
+#include "components/prefs/pref_filter.h"
 
 class PrefFilter;
 
 namespace base {
 class Clock;
 class DictionaryValue;
 class FilePath;
 class HistogramBase;
 class JsonPrefStoreCallbackTest;
 class JsonPrefStoreLossyWriteTest;
@@ skipping 20 matching lines @@
   // to disk.
   using OnWriteCallbackPair =
       std::pair<base::Closure, base::Callback<void(bool success)>>;
 
   // Returns instance of SequencedTaskRunner which guarantees that file
   // operations on the same file will be executed in sequenced order.
   static scoped_refptr<base::SequencedTaskRunner> GetTaskRunnerForFile(
       const base::FilePath& pref_filename,
       base::SequencedWorkerPool* worker_pool);
 
-  // |sequenced_task_runner| must be a shutdown-blocking task runner, ideally
-  // created by the GetTaskRunnerForFile() method above.
-  // |pref_filename| is the path to the file to read prefs from.
-  JsonPrefStore(
-      const base::FilePath& pref_filename,
-      const scoped_refptr<base::SequencedTaskRunner>& sequenced_task_runner,
-      std::unique_ptr<PrefFilter> pref_filter);
+  // |pref_filename| is the path to the file to read prefs from. It is incorrect
+  // to create multiple JsonPrefStore with the same |pref_filename|.
+  // |sequenced_task_runner| is used for asynchronous reads and writes. It must
+  // have the base::TaskShutdownBehavior::BLOCK_SHUTDOWN and base::MayBlock()
+  // traits. Unless external tasks need to run on the same sequence as
+  // JsonPrefStore tasks, keep the default value.
+  JsonPrefStore(const base::FilePath& pref_filename,
+                scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner =
+                    base::CreateSequencedTaskRunnerWithTraits(
+                        {base::MayBlock(),
+                         base::TaskShutdownBehavior::BLOCK_SHUTDOWN}),
+                std::unique_ptr<PrefFilter> pref_filter = nullptr);
 
   // PrefStore overrides:
   bool GetValue(const std::string& key,
                 const base::Value** result) const override;
   std::unique_ptr<base::DictionaryValue> GetValues() const override;
   void AddObserver(PrefStore::Observer* observer) override;
   void RemoveObserver(PrefStore::Observer* observer) override;
   bool HasObservers() const override;
   bool IsInitializationComplete() const override;
 
@@ skipping 161 matching lines @@
 
   bool has_pending_write_reply_ = true;
   base::Closure on_next_successful_write_reply_;
 
   WriteCountHistogram write_count_histogram_;
 
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };
 
 #endif  // COMPONENTS_PREFS_JSON_PREF_STORE_H_