[usecase] Move CSSStyleAttribute to usecase-webapi-xwalk-tests

- usecase-webapi-w3c-tests have been renamed to usecase-webapi-xwalk-tests, move CSSStyleAttribute to there
Merge pull request #1368 from haoxli/usecase/move-cssstyle

[usecase] Move CSSStyleAttribute to usecase-webapi-xwalk-tests