This reverts commit adf0fc8c.

Original change's description:
> [heap] Tune incremental marking step size.
>
> The main thread now can reduce marking step size if concurrent marking
> tasks are making progress and the bailout worklist is empty.
>
> Bug: chromium:694255

Change-Id: Ib2f04be258e14887059d88da301ddf17f6b453b4
Reviewed-on: https://chromium-review.googlesource.com/794135Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49685}