Revert of [heap] Cancel tasks before tearing down the heap. (patchset #2 id:20001 of https://codereview.chromium.org/2413203004/ )

Reason for revert:
Timeout on gc stress bot.

Original issue's description:
> [heap] Cancel tasks before tearing down the heap.
>
> BUG=chromium:654343
>
> Committed: https://crrev.com/9365463d6f1fc11b2369fff63ca0ac825c61eae4
> Cr-Commit-Position: refs/heads/master@{#40272}

TBR=mlippautz@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:654343

Review-Url: https://codereview.chromium.org/2414073002
Cr-Commit-Position: refs/heads/master@{#40279}