WebSetting plugin implementation

WebSetting plugin implementation

The Web setting extension defines APIs that manages the setting states of the Web view in Web application.
Properties of the Web view that can be managed via Web setting API:
- Delete all cookies saved for the web view in the Web application
- Set a custom user agent string of the web view in Web application

This commit includes implementation of API for Web application to set custom user agent and
remove all application cookies.

Result of running tct-tizen-websetting tests: 22/22 pass.

This pull request is dependant on: https://github.com/crosswalk-project/crosswalk/pull/2392

BUG=XWALK-1063
In previous commit I changed it to "std::istringstream buf(common::Extension::GetRuntimeVariable("app_id", 64));".
It was a good idea to initialize buf directly but I can't pass std::istringstream to 
picojson::parse() directly as it takes a reference to std::istringstream as a parameter, when I would change it as suggested, it would be passing rvalue as a non-const reference argument and compiler wouldn't be able to build it as it can not convert std::istringstream to std::istringstream&.