Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/lib/BUILD.gn

Index: services/preferences/public/cpp/lib/BUILD.gn
diff --git a/services/preferences/public/cpp/lib/BUILD.gn b/services/preferences/public/cpp/lib/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..a66e2b8b049476ad95e40396623421dd07fb19d9
--- /dev/null
+++ b/services/preferences/public/cpp/lib/BUILD.gn
@@ -0,0 +1,16 @@
+# 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.
+
+source_set("lib") {
+  visibility = [ "//services/preferences/*" ]
+
+  sources = [
+    "util.cc",
+    "util.h",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}