Object.is should use SameValue

BUG=v8:3576
LOG=
R=arv@chromium.org, svenpanne@chromium.org, wingo@igalia.com

Review URL: https://codereview.chromium.org/626323002

Patch from Diego Pino <dpino@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24658 ce2b1a6d-e550-0410-aec6-3dcde31c8c00