Support revised shadow-piercing descendant combinator

[addyosmani]

Per recent spec changes, >>>, shadow piercing combinator, is the new name for /deep/ combinator. Blink has already shipped a /deep/ combinator. We have to rename it. I propose that to avoid breaking things in the interim we continue to support both /deep/and >>>.

This brings us in line with https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/hRw781MV3mE

[addyosmani]

I have a local patch for this but am avoiding a PR while there's uncertainty in Blink about us continuing with /deep/ and >>>.

[TimvdLippe]

This selector has been removed.