The threading test failures seem to be due to a GC-unsafe place, that
mixed handles and raw pointers, in the runtime code for context
lookup.

R=fschneider@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/7890002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9356 ce2b1a6d-e550-0410-aec6-3dcde31c8c00