https://bugs.webkit.org/show_bug.cgi?id=120223

Reviewed by Anders Carlsson.

* wtf/RefPtr.h: Switch to copy-and-swap in the assignment operators. The move ones already were done
that way.
* wtf/RetainPtr.h: Ditto, except for the move ones too.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154519 268f45cc-cd09-0410-ab3c-d52691b4dbfc