It could happen that we shrink a live object on the main thread (e.g.
MigrateFastToSlow) while we're sweeping the same page. The main
thread first creates a filler object that the release-stores the new
map. Therefore it's important to barrier load the map word of live
objects from the sweeper thread.

BUG=none
R=ulan@chromium.org,hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/797623002

Cr-Commit-Position: refs/heads/master@{#25780}