Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/lib/util.h

+// Copyright 2017 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 SERVICES_PREFERENCES_PUBLIC_CPP_LIB_UTIL_H_
+#define SERVICES_PREFERENCES_PUBLIC_CPP_LIB_UTIL_H_
+
+#include <memory>
+#include <vector>
+
+#include "base/strings/string_piece.h"
+
+namespace base {
+class DictionaryValue;
+class Value;
+}
+
+namespace prefs {
+
+void SetValue(base::DictionaryValue* dictionary_value,
+              const std::vector<base::StringPiece>& path_components,
+              std::unique_ptr<base::Value> value);
+
+}  // namespace prefs
+
+#endif  // SERVICES_PREFERENCES_PUBLIC_CPP_LIB_UTIL_H_