Object.observe: avoid accessing acceptList properties more than once

BUG=v8:3315
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/270763003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21244 ce2b1a6d-e550-0410-aec6-3dcde31c8c00