Move start of incremental marking out of
RefillLinearAllocationAreaFromFreeList. This avoids a potential
safepoint while holding allocation_mutex_.

Bug: v8:10315
Change-Id: Ieb60ac68f26199eea7b6b7ad6d874851382f3d69
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2287496
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68751}