This patch removes JSFunction::SetInstancePrototype() from
JSFunction's public API and makes it an implementation detail
of SetPrototype().

Also clear out constructor field of JSFunction Map when
transitioning from non-instance prototype to instance
prototype.

Change-Id: If51d37bf6047b51b934d1b370fb52bb5cf5ffed4
Reviewed-on: https://chromium-review.googlesource.com/483961Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44821}