shadow-dom: Added tests that checks inheritance of contentEditable

Added tests of inheritance of contentEditable of shadow host.
This tests is related to the 'Editing' sections of User Interaction of
the specs.
http://www.w3.org/TR/2013/WD-shadow-dom-20130514/#editing
Merge pull request #140 from summerwind/submission/summerwind

shadow-dom: Added tests that checks inheritance of contentEditable