Revert of Revert of Promise assimilation fix. (patchset #1 id:1 of https://codereview.chromium.org/1176163004/)

Reason for revert:
Test failures are bogus. Snapshot blob and natives blob are out of sync due to build being weird.

Original issue's description:
> 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
>
> Committed: https://crrev.com/5bb75f514027f79303396dba823c2d78c6add83b
> Cr-Commit-Position: refs/heads/master@{#28927}

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/1181533006

Cr-Commit-Position: refs/heads/master@{#28928}