Make /deep/ behave like the descendant combinator " " in CSS live profile; effectively no-op.

See "Intent to Remove" thread for more details: groups.google.com/a/chromium.org/d/topic/blink-dev/HX5Y8Ykr5Ns/discussion

w3.org/TR/selectors-4/#profiles
CSS live profile = CSS selector used in CSS file or inside of