ReduceToString performs heap access on the background thread.

Change-Id: Ic93ee21d6eeb96fe1dbd2b98043e6ccff0eefa64
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2577458
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71648}