This makes sure that we only record relocation slots for code target
patches that happen in marked objects. Unmarked ones might be visited
again, whereas marked ones are alive and will not be visited again.

R=ulan@chromium.org
BUG=chromium:152615,chromium:144230

Review URL: https://codereview.chromium.org/11040021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12655 ce2b1a6d-e550-0410-aec6-3dcde31c8c00