This CL optimizes Heap::EnsureSweepingCompleted() by only ensure that
sweeping is finished for that object's page only and not for the whole
heap.

For this purpose the page is removed from the sweeping_list_ and
processed on the main thread. In case the object is in new space,
this method will just use EnsureIterabilityCompleted() since
the iterability task doesn't currently have any kind of synchronization.
Also the new space will generally be much smaller.

Bug: v8:11837
Change-Id: I7e878b5c84c223009fac0d58798197be5cd524e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2958488
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75148}