This keeps dying maps alive for FLAG_retain_maps_for_n_gc garbage collections
to increase chances of them being reused for new objects in future and
decrease number of deoptimizations.
BUG=v8:3664
LOG=N
TEST=cctest/test-heap/MapRetaining
Review URL: https://codereview.chromium.org/980523004
Cr-Commit-Position: refs/heads/master@{#27040}
