Revert of [d8] bounds-check before getting Shell::Worker internal field (patchset #4 id:80001 of https://codereview.chromium.org/1214053004/)

Reason for revert:
[Sheriff] Fails here:
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20shared/builds/4737

Original issue's description:
> [d8] bounds-check before getting Shell::Worker internal field
>
> Prevents fatal error in debug builds
>
> BUG=v8:4271
> R=binji@chromium.org
> LOG=N
>
> Committed: https://crrev.com/43ce9c6f101c4224addd9a54e0c39963188dc7fa
> Cr-Commit-Position: refs/heads/master@{#29524}

TBR=binji@chromium.org,jochen@chromium.org,adamk@chromium.org,caitpotter88@gmail.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4271

Review URL: https://codereview.chromium.org/1215333012

Cr-Commit-Position: refs/heads/master@{#29525}