Disable tracking of double fields during snapshot creation.

Follow-up to r18298.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/101123004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18909 ce2b1a6d-e550-0410-aec6-3dcde31c8c00