Use InitialPropertiesLength to reinitialize rather than local calculation.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/256973003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21004 ce2b1a6d-e550-0410-aec6-3dcde31c8c00