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

Author: SimonSapin

selectors/Overview.bs

@@ -3175,7 +3175,7 @@ Grammar</h2>
 
 		combinator
 			/* combinators can be surrounded by whitespace */
-			: S+ | S* [ '>' | '+' | '~' | COLUMN ] S*
+			: S+ | S* [ '>' | '+' | '~' | COLUMN | '/' IDENT '/' ] S*
 			;
 
 		compound_selector_list