implement NPN_UserAgent, which was trickier than expected

dom/plugins/PluginModuleParent.cpp

@@ -342,6 +342,14 @@ PluginModuleParent::NPP_SetValue(NPP instance, NPNVariable variable,
     return InstCast(instance)->NPP_SetValue(variable, value);
 }
 
+bool
+PluginModuleParent::AnswerNPN_UserAgent(nsCString* userAgent)
+{
+    NPP_t dummy = { 0, 0 };
+    *userAgent = NullableString(mNPNIface->uagent(&dummy));
+    return true;
+}
+
 bool
 PluginModuleParent::RecvNPN_GetStringIdentifier(const nsCString& aString,
                                                 NPRemoteIdentifier* aId)