... when it is moved or changed by GC.

Port r9697 (d53382a)

BUG=
TEST=

Review URL: http://codereview.chromium.org/8362024
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9740 ce2b1a6d-e550-0410-aec6-3dcde31c8c00