Native setters (see AccessorInfo in accessors.h) didn't have the ability
to return a result value. As a consequence of this, for instance, Reflect.set
on the length property of arrays had the wrong behavior:

var y = [];
Object.defineProperty(y, 0, {value: 42, configurable: false})
Reflect.set(y, 'length', 0)

The Reflect.set call used to return true. Now it returns false as
required by the spec.

BUG=v8:5401

Review-Url: https://codereview.chromium.org/2397603003
Cr-Commit-Position: refs/heads/master@{#40579}