This prepares ground for switching mark-compactor to use
Worklist data-structure instead of the existing marking deque.

BUG=chromium:694255

Change-Id: I0ac4c563018a9619962fb4bf388b5f3cceffb86d
Reviewed-on: https://chromium-review.googlesource.com/544933Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46178}