Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

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

+// 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.
+
+#include "services/preferences/public/cpp/lib/util.h"
+
+#include <utility>
+
+#include "base/memory/ptr_util.h"
+#include "base/values.h"
+
+namespace prefs {
+
+void SetValue(base::DictionaryValue* dictionary_value,
+              const std::vector<base::StringPiece>& path_components,
+              std::unique_ptr<base::Value> value) {
+  for (size_t i = 0; i < path_components.size() - 1; ++i) {
+    if (!dictionary_value->GetDictionaryWithoutPathExpansion(
+            path_components[i], &dictionary_value)) {
+      auto new_dict_value_owner = base::MakeUnique<base::DictionaryValue>();
+      auto* new_dict_value = new_dict_value_owner.get();
+      dictionary_value->SetWithoutPathExpansion(
+          path_components[i], std::move(new_dict_value_owner));
+      dictionary_value = new_dict_value;
+    }
+  }
+  const auto& key = path_components.back();
+  if (value)
+    dictionary_value->SetWithoutPathExpansion(key, std::move(value));
+  else
+    dictionary_value->RemoveWithoutPathExpansion(key, nullptr);
+}
+
+}  // namespace prefs