NativeContext::map_cache reference should be strong in heap snapshots

This change makes the reference itself a strong one. We will also need a special handling to make references from MapCache object to Map weak in the heap snapshots. The latter will be addressed separately.

BUG=chromium:357060
LOG=Y
R=alph@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/214683002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20304 ce2b1a6d-e550-0410-aec6-3dcde31c8c00