This reverts commit 2690e2fc.

Reason for revert: this is not needed because objects in the worklist
are guaranteed to be not black allocated.

Original change's description:
> [heap] Correctly check for black allocated objects in concurrent marker.
> 
> The markbit check should be performed before using the map of the
> object.
> 
> Change-Id: Ia19e48fd4660387d239e1e330368808727359c7f
> Reviewed-on: https://chromium-review.googlesource.com/c/1301496
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57040}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org

Change-Id: I4f188197620c511060fda4f60c80a3c389007054
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/1301993Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57043}