Original issue's description:
> [es6] Fix Function and GeneratorFunction built-ins subclassing.
>
> BUG=v8:3101, v8:3330
> LOG=Y
>
> Committed: https://crrev.com/99e7f872d3d0a5fb799dcbafb05537cda491314a
> Cr-Commit-Position: refs/heads/master@{#31708}

The problem was in another CL, this is a clean reland with improved tests.

BUG=v8:3101, v8:3330
LOG=Y

Review URL: https://codereview.chromium.org/1415683007

Cr-Commit-Position: refs/heads/master@{#31756}