LiveObjectIterator sometimes runs concurrently to the main thread. In this
scenarios we are not allowed to access memory of live objects in non-atomic
ways. Use synchronized reads where needed.

Correctness (already ok in current state):
- Reading a larger size is fine per definition.
- Reading a smaller size is fine since are guaranteed that one word fillers will
  follow.

BUG=v8:5583
R=ulan@chromium.org,hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2477823003
Cr-Commit-Position: refs/heads/master@{#40798}