This fixes a corner case where the main thread has items in the local
segments but the global pool is empty. In such case concurrent marking
tasks are not posted and marking is performed on the main thread.

Bug: chromium:934453
Change-Id: Ic34cd4ecb59b848021d8d8b086904b415669f5e6
Reviewed-on: https://chromium-review.googlesource.com/c/1482739Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59802}