WebSetting plugin implementation - xwalk part

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 setting and reading custom user agent string and
removal of application cookies on crosswalk engine side.

Result of running tct-tizen-websetting tests after this change and related change in crosswalk extensions: 22/22 tests pass.

This change concerns XWALK-1063 issue.
As I see from the implementation, CookieManager does not have to be Application's data member, it is more like a strategy class (just does some functionality with a given context) and it does not store any thing itself. 
So, can we do the following:
// In the place when it is needed (client code).
CookieManager cookie_manager(application, runtime_context); 
cookie_manager.SetUserAgentString(); // or cookie_manager.RemoveAllCookies();