We do not need to scan main thread handles when starting incremental
marking. This reduces the time to start incremental marking.

Bug: v8:11645
Change-Id: Ib99a13e7875f50fbfe5346ac0e186d8960ea1337
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2826124Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73994}