This is based on https://chromiumcodereview.appspot.com/9117034/

Doesn't have much impact on its own, but is the basis for Ulan's CL https://chromiumcodereview.appspot.com/9117034/, which moves the logic to C++.

R=ulan@chromium.org
BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/9307083

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10982 ce2b1a6d-e550-0410-aec6-3dcde31c8c00