Revert e9661dc2f646: [selectors] Remove reference combinator form the grammar

This syntax is now genaralized and used for named combinators,
the first of which is /deep/:
http://dev.w3.org/csswg/css-scoping/#deep-combinator

--HG--
extra : rebase_source : efe8e3010c9112c538633492aab13854e733e98c