Unreviewed, rolling out r191815 and r191821.

Unreviewed, rolling out r191815 and r191821.
https://bugs.webkit.org/show_bug.cgi?id=150781

Seems to have broken JSC API tests on some platforms
(Requested by ap on #webkit).

Reverted changesets:

"[ES6] Add support for toStringTag"
https://bugs.webkit.org/show_bug.cgi?id=150696
http://trac.webkit.org/changeset/191815

"Unreviewed, forgot to mark tests as passing for new feature."
http://trac.webkit.org/changeset/191821

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191863 268f45cc-cd09-0410-ab3c-d52691b4dbfc