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