Avoid unnecessary data race on FLAG_track_double_fields

BUG=none
R=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/639723002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24456 ce2b1a6d-e550-0410-aec6-3dcde31c8c00