This fixes a CHECK failure in MapVerify, and gets the correct behaviour
for uses of the well-known symbols.

BUG=v8:7611, chromium:866229
R=jkummerow@chromium.org, mvstanton@chromium.org, bmeurer@chromium.org

Change-Id: I5d679357b8807ea9d1054121d8d336fe0dd43c7c
Reviewed-on: https://chromium-review.googlesource.com/1162278Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#54905}