Version 3.27.18 (based on bleeding_edge revision r21612)

Remove PROHIBITS_OVERWRITING as it is subsumed by non-configurable properties.

Performance and stability improvements on all platforms.

git-svn-id: https://v8.googlecode.com/svn/trunk@21613 ce2b1a6d-e550-0410-aec6-3dcde31c8c00