In ShillClientHelper use a{sv} for property dictionaries

chromeos/dbus/shill_client_helper.h

Index: chromeos/dbus/shill_client_helper.h
diff --git a/chromeos/dbus/shill_client_helper.h b/chromeos/dbus/shill_client_helper.h
index a52fc222eae758b43c86d32bd1b97554f9f044db..c615a3e74e095be78816af2a15378ad2d1dc943e 100644
--- a/chromeos/dbus/shill_client_helper.h
+++ b/chromeos/dbus/shill_client_helper.h
@@ -141,15 +141,23 @@ class ShillClientHelper {
 
   const dbus::ObjectProxy* object_proxy() const { return proxy_; }
 
-  // Appends the value (basic types and string-to-string dictionary) to the
-  // writer as a variant.
+  // Appends the value to the writer as a variant. If |value| is a Dictionary it
+  // will be written as a string -> varient dictionary, a{sv}. If |value| is a
+  // List then it must be a List of String values and is writen as type 'as'.
   static void AppendValueDataAsVariant(dbus::MessageWriter* writer,
                                        const base::Value& value);
 
-  // Appends a string-to-variant dictionary to the writer.
-  static void AppendServicePropertiesDictionary(
-      dbus::MessageWriter* writer,
-      const base::DictionaryValue& dictionary);
+  // As above but if |use_string_dictionaries| is true and |value| is a
+  // Dictionary then it will be wtitten as type 'a{ss}'. (This is to support
+  // Cellular.APN which expects a string -> string dictionary).
+  static void AppendValueDataAsVariantEx(dbus::MessageWriter* writer,
+                                         const base::Value& value,
+                                         bool use_string_dictionaries);
+
+  // Appends a string-to-variant dictionary to the writer as an '{sv}' array.
+  // Each value is written using AppendValueDataAsVariant.
+  static void AppendServicePropertiesDictionary(dbus::MessageWriter* writer,
+                                                const base::DictionaryValue&);
 
  protected:
   // Reference / Ownership management. If the number of active refs (observers