This fixes processing of WeakMaps so that keys on evacuation candidates
which are also reachable by other strong paths are correctly recorded in
the slots buffer.

Also backing stores that reside in the large-object-space now use the
correct anchor slot.

R=vegorov@chromium.org
BUG=v8:2060
TEST=cctest/test-weakmaps/Regress2060b

Review URL: https://chromiumcodereview.appspot.com/10034018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11279 ce2b1a6d-e550-0410-aec6-3dcde31c8c00