Those counters will be incremented concurrently during parallel evacuation of
newspace. This change makes sure we do the updates atomically.

Note that actual reasoning about them happens sequentially on the main thread.

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

Review URL: https://codereview.chromium.org/1585843010

Cr-Commit-Position: refs/heads/master@{#33326}