Put new global var semantics behind a flag until WebKit tests are cleaned up.

Enabled stepping into callback passed to builtins. (Chromium issue 109564)

Performance and stability improvements on all platforms.

git-svn-id: http://v8.googlecode.com/svn/trunk@11413 ce2b1a6d-e550-0410-aec6-3dcde31c8c00