Currently the incremental marking visitor treats elements of normalized
map caches weakly by coloring the caches grey without pusing to marking
deque.

The mark-compact prologue then clears all normalized map caches.

We can achieve similar effect by just clearing the caches in the marking
visitor.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2745183002
Cr-Commit-Position: refs/heads/master@{#43941}