Currently string and preparse data trimming code creates filler object
without clearing the slots in the trimmed area. This currently works
because the slots are overwritten by filler/free space map.

This CL explicitly clears the slots and makes the code more robust.

Bug: v8:9454
Change-Id: I20ad8a210eb17932e46be5df4b42389955b5e5eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1778023Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63481}