Clearing out the constructor field is invalid in the case where the
function's map has transitioned since the last SetPrototype call.

Bug: chromium:714972
Change-Id: Ie918702a128219c4995b805f7c9a53b41cc4e4b6
Reviewed-on: https://chromium-review.googlesource.com/486130
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44906}