HeapWithPointerCompressionTest.HeapLayout sometimes raced with
background threads. When iterating chunks, background threads might add
chunks concurrently. Prevent this by enforcing safepoint during
iteration.

Bug: v8:10315
Change-Id: I2852ec3ea18905b669e60eadc660d9343ea35f48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390767
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69696}