brillo: handle arrays of string maps in dbus serialization

Bug: 26778228
TEST=compiles on chromeos, manual testing of dbus serialization

Change-Id: I3876c4f0263add5d06732af2dc0c05d1b9f4dd54
diff --git a/brillo/dbus/data_serialization.cc b/brillo/dbus/data_serialization.cc
index 5c1d50e..86d1c63 100644
--- a/brillo/dbus/data_serialization.cc
+++ b/brillo/dbus/data_serialization.cc
@@ -211,6 +211,9 @@
     return PopTypedMapFromReader<std::string, std::string>(reader, value);
   else if (signature == "a{sv}")
     return PopTypedValueFromReader<brillo::VariantDictionary>(reader, value);
+  else if (signature == "aa{ss}")
+    return PopTypedArrayFromReader<
+        std::map<std::string, std::string>>(reader, value);
   else if (signature == "aa{sv}")
     return PopTypedArrayFromReader<brillo::VariantDictionary>(reader, value);
   else if (signature == "a{sa{ss}}")