Revert of VariableProxy: when cloning, don't even think about creating dangling references. (patchset #1 id:1 of https://codereview.chromium.org/2368253002/ )

Reason for revert:
Needed for  https://codereview.chromium.org/2373443003/

Original issue's description:
> VariableProxy: when cloning, don't even think about creating dangling references.
>
> The code path for cloning resolved VariableProxys (into a different
> Zone) was never hit, but if it was, it would create a dangling
> reference, since the Variable would stay in the original Zone.
>
> Kudos to verwaest@ for finding this!
>
> R=verwaest@chromium.org
> BUG=
>
> Committed: https://crrev.com/fd429bdb9e70cb8c4f8a4bbef0806e008c60440c
> Cr-Commit-Position: refs/heads/master@{#39723}

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

Review-Url: https://codereview.chromium.org/2368303002
Cr-Commit-Position: refs/heads/master@{#39726}