This is a fix that removes a left over from
  https://crrev.com/4566531c6e593ae8bffc2ef96f82c91d15a68acf

With this flag set to false still present, we would race on inserting in the
local old-to-new remembered set.

R=ulan@chormium.org
BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1783283003

Cr-Commit-Position: refs/heads/master@{#34728}