Now there are two functions, one corresponding to the spec's
[[PreventExtensions]] and one corresponding to Object.preventExtensions.
They differ in what they return.

This CL is in preparation of implementing Reflect.preventExtensions.

R=rossberg
BUG=

Review URL: https://codereview.chromium.org/1377103005

Cr-Commit-Position: refs/heads/master@{#31096}