BUG=
TEST=cctest/test-api/IndependentWeakHandle

Review URL: https://codereview.chromium.org/11369131
Patch from Kentaro Hara <haraken@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00