This ensures that incremental marking step does not change the top and limit
pointers of the old space, which is needed for allocation folding.

For more info see:
https://bugs.chromium.org/p/chromium/issues/detail?id=659165#c13

BUG=chromium:659165

Review-Url: https://codereview.chromium.org/2469273002
Cr-Commit-Position: refs/heads/master@{#40720}