Version 3.27.4 (based on bleeding_edge revision r21317)

Drop thenable coercion cache (Chromium issue 372788).

Skip write barriers when updating the weak hash table (Chromium issue 359401).

Performance and stability improvements on all platforms.

git-svn-id: https://v8.googlecode.com/svn/trunk@21318 ce2b1a6d-e550-0410-aec6-3dcde31c8c00