Revert of mjsunit: Fix the error message produced by assertInstanceof. (patchset #1 id:1 of https://codereview.chromium.org/2413153004/ )

Reason for revert:
This is probably not working for falsey values. I cannot JavaScript :P

Original issue's description:
> mjsunit: Fix the error message produced by assertInstanceof.
>
> Used to be:
>
> Failure: expected <foo> is not an instance of <Bar> but of < Baz>> found <undefined>
>
> Should be:
>
> Failure: <foo> is not an instance of <Bar> but of <Baz>
>
> BUG=
>
> Committed: https://crrev.com/2a480eff395756f36eb0ae2fc0a573454b394268
> Cr-Commit-Position: refs/heads/master@{#40319}

TBR=verwaest@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/2421033003
Cr-Commit-Position: refs/heads/master@{#40331}