When we are not going to be accessing the heap anymore, we can park the
LocalHeap which signals to not wait for this thread when requesting
safepoints.

There are a couple of places where we want to explicitly allow access
to the heap, even though we have previously parked. We use
UnparkedScope for those cases.

Bug: v8:7790
Change-Id: Ic0acc51fe02af89836226670b828db4aafba4d0e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2319993
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69173}