Reland of [heap] Fix clearing slots for map and size during LeftTrimFixedArray (patchset #1 id:1 of https://codereview.chromium.org/1947473002/ )

Reason for revert:
Unrelated to waterfall failures.

Original issue's description:
> Revert of [heap] Fix clearing slots for map and size during LeftTrimFixedArray (patchset #1 id:1 of https://codereview.chromium.org/1943263002/ )
>
> Reason for revert:
> GC-stress failures: https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/3305/steps/Mjsunit/logs/array-copywithin
>
> Original issue's description:
> > [heap] Fix clearing slots for map and size during LeftTrimFixedArray
> >
> > BUG=chromium:598319
> > LOG=N
> >
> > Committed: https://crrev.com/5aa1d9c3cfde4389fb5a34e98a5dccbca9dc9f12
> > Cr-Commit-Position: refs/heads/master@{#35970}
>
> TBR=ulan@chromium.org,mlippautz@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=chromium:598319
>
> Committed: https://crrev.com/a4129bb45a3bb8cc68bb2e07ee1e0a734f04ce4b
> Cr-Commit-Position: refs/heads/master@{#35979}

TBR=ulan@chromium.org,yangguo@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:598319

Review-Url: https://codereview.chromium.org/1947503002
Cr-Commit-Position: refs/heads/master@{#35986}