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

Review URL: https://codereview.chromium.org/1098663002

Cr-Commit-Position: refs/heads/master@{#28926}