Introduce a new framework for back-and-forth tracked/protected preferences migration.

chrome/browser/prefs/tracked/tracked_preferences_migration.h

+// 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.
+
+#ifndef CHROME_BROWSER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_
+#define CHROME_BROWSER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_
+
+#include <set>
+#include <string>
+
+class JsonPrefStore;
+
+// Sets up JsonPrefStore::OnFileReadInterceptors on |unprotected_pref_store|
+// and |protected_pref_store| such that they are only handed back to their
+// respective JsonPrefStores on read after we've made sure that the most recent
+// pref value for |unprotected_pref_names| are in the |unprotected_pref_store|
+// and conversely for protected preferences.
+// NOTE: In the event of a synchronous read request (i.e.,
+// JsonPrefStore::ReadPrefs()): the JsonPrefStore::OnFileReadInterceptors will
+// make the first read asynchronous, but the double read will complete
+// synchronously on the second ReadPrefs().
+void SetupTrackedPreferencesMigration(
+    const std::set<std::string>& unprotected_pref_names,
+    const std::set<std::string>& protected_pref_names,
+    JsonPrefStore* unprotected_pref_store,
+    JsonPrefStore* protected_pref_store);
+
+#endif  // CHROME_BROWSER_PREFS_TRACKED_TRACKED_PREFERENCES_MIGRATION_H_