The April 14 2015 final draft of the ES6 specification states that the
`prototype` property of generator function instances should be writable.

BUG=v8:4140, v8:4140
LOG=N
R=arv@chromium.org

Review URL: https://codereview.chromium.org/1153633003

Cr-Commit-Position: refs/heads/master@{#28641}