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.
this class is looking just like wrapping around two variables:
bool user_agent_overriden_;
std::string overriden_user_agent_string_;

I've two questions:
1) can we drop "user_agent_overriden_" and use "overriden_user_agent_string_.empty()" instead?
2) can we drop XWalkUserAgentUtil and simply use overriden_user_agent_string_ in the client code?