One doesn't have to update remembered set if it already contains a mark for
the given address.

BUG=39766

Original review: http://codereview.chromium.org/1603002

Review URL: http://codereview.chromium.org/1577005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00