Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

components/prefs/writeable_pref_store.cc

Index: components/prefs/writeable_pref_store.cc
diff --git a/components/prefs/writeable_pref_store.cc b/components/prefs/writeable_pref_store.cc
new file mode 100644
index 0000000000000000000000000000000000000000..249ed542430c29972ad61eb2549ca526b85120f0
--- /dev/null
+++ b/components/prefs/writeable_pref_store.cc
@@ -0,0 +1,12 @@
+// 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 "components/prefs/writeable_pref_store.h"
+
+void WriteablePrefStore::ReportSubValuesChanged(
+    const std::string& key,
+    std::set<std::vector<std::string>> path_components,
+    uint32_t flags) {
+  ReportValueChanged(key, flags);
+}