Add checks to the marking queue

Add checks to the marking queue

Hopefully we'll catch heap corruption earlier where identifying the
object that holds a stale pointer.

Speaking of staleness, also remove old debugging code.

BUG=chromium:128415
R=ulan@chromium.org,hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/843013005

Cr-Commit-Position: refs/heads/master@{#26095}