This reverts commit 0d06e42b.

Reason for revert: clusterfuzz and canary crashes.

BUG=chromium:729209,v8:6456

Original change's description:
> [heap] Use partial free when shrinking instead of uncommitting
> 
> This fixes the counter inconsistencies while leaving the memory in an
> inaccessible state.
> 
> Bug: chromium:724947
> Change-Id: I431eb6fda84922a52dfb9380c6b482ada55bccee
> Reviewed-on: https://chromium-review.googlesource.com/519164
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#45647}

TBR=hpayer@chromium.org,mlippautz@chromium.org
# Not skipping CQ checks because original CL landed > 1 day ago.
Bug: chromium:724947

Change-Id: I6c52b478b89a858ba984fe17f86cdf15fcfa974c
Reviewed-on: https://chromium-review.googlesource.com/525716Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45733}