Issue 5039 - add support of nullable non-object values in settings

src/JsEngine.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 209 matching lines @@
   const JsContext context(shared_from_this());
   return JsValuePtr(new JsValue(shared_from_this(), v8::Boolean::New(val)));
 }
 
 AdblockPlus::JsValuePtr AdblockPlus::JsEngine::NewObject()
 {
   const JsContext context(shared_from_this());
   return JsValuePtr(new JsValue(shared_from_this(), v8::Object::New()));
 }
 
+JsValuePtr JsEngine::NullValue()
+{
+  const JsContext context(shared_from_this());
+  return JsValuePtr(new JsValue(shared_from_this(), v8::Null()));
+}
+
 AdblockPlus::JsValuePtr AdblockPlus::JsEngine::NewCallback(
     v8::InvocationCallback callback)
 {
   const JsContext context(shared_from_this());
 
   // Note: we are leaking this weak pointer, no obvious way to destroy it when
   // it's no longer used
   std::weak_ptr<JsEngine>* data =
       new std::weak_ptr<JsEngine>(shared_from_this());
   v8::Local<v8::FunctionTemplate> templ = v8::FunctionTemplate::New(callback,
@@ skipping 89 matching lines @@
 
 
 void AdblockPlus::JsEngine::SetGlobalProperty(const std::string& name, 
                                               AdblockPlus::JsValuePtr value)
 {
   auto global = GetGlobalObject();
   if (!global)
     throw std::runtime_error("Global object cannot be null");
   global->SetProperty(name, value);
 }