Made "map_cache" a weak field of global context and added a pass over
all caches late in the marking phase.

R=vegorov@chromium.org
BUG=v8:1516
TEST=cctest/test-api/Regress1516

Review URL: http://codereview.chromium.org/7285031

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8515 ce2b1a6d-e550-0410-aec6-3dcde31c8c00