Rename CreateFillerObjectFromSweeper to CreateFillerObjectAtBackground.
Also use CreateFillerObjectAtBackground in PagedSpace::Free since this
is used from both main and background threads.

Bug: v8:10315
Change-Id: I1dc8ca2b1c81bdfd192c0ae8d8547eb577f3f8c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235534
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68255}