Revert of Promise assimilation fix. (patchset #8 id:160001 of https://codereview.chromium.org/1098663002/)

Reason for revert:
Test failures: https://chromegw.corp.google.com/i/client.v8/builders/V8%20Linux64/builds/3829

Original issue's description:
> Promise assimilation fix.
>
> Let x be a fulfilled promise and y be another promise. |x.then(() => y)|
> should call |y.then|, but the current implementation calls PromiseChain.
> We can see the difference when we set a custom function to |y.then|.
>
> This CL fixes the spec violation, but as a result |then| is no longer
> a wrapper of |chain| and in some cases it does not work well with
> |accept| or |chain|. That is not a problem for ES6 promise users because
> ES6 promise doesn't have them.
>
> LOG=N
> BUG=477921
>
> Committed: https://crrev.com/2f57dff3ea0c45e1a61b334fda962460f89d71bc
> Cr-Commit-Position: refs/heads/master@{#28926}

TBR=arv@chromium.org,caitpotter88@gmail.com,rossberg@chromium.org,yhirano@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=477921

Review URL: https://codereview.chromium.org/1176163004

Cr-Commit-Position: refs/heads/master@{#28927}