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
My question is `common::Extension::GetRuntimeVariable()` return `std::string` directly, isn't the value you want? I greped the exiting codo, seems most places are same as you does(using istringstream and picojson to parse). But one exception on [common/virtual_fs.cc#L195] (https://github.com/crosswalk-project/tizen-extensions-crosswalk/blob/master/common/virtual_fs.cc#L195) . 
```
std::string id_str = common::Extension::GetRuntimeVariable("app_id", 64);
std::string pkg_id = id_str.substr(1, id_str.rfind('"') - 1);
```

I'm not familiar with this function, @tmpsantos do you have any suggestion?