Move some code from Runtime_SetPrototype to JSObject::SetPrototype.

Move some code from Runtime_SetPrototype to JSObject::SetPrototype.

This is in preparation of implementing Reflect.setPrototypeOf.

R=verwaest@chromium.org
BUG=

Committed: https://crrev.com/d603dc5c42e8031796c1c2873928ca98cdd24dc2
Cr-Commit-Position: refs/heads/master@{#31432}

[Toon Verwaest, 2015-10-20 15:13:32]

lgtm

[neis, 2015-10-21 09:30:08]

The CQ bit was checked by neis@chromium.org

[neis, 2015-10-21 09:30:08]

The patchset sent to the CQ was uploaded after l-g-t-m from
verwaest@chromium.org
Link to the patchset: https://codereview.chromium.org/1409003005/#ps20001
(title: "Rebase.")

[commit-bot: I haz the power, 2015-10-21 09:30:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1409003005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1409003005/20001

[commit-bot: I haz the power, 2015-10-21 09:54:26]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-21 09:54:46]

Patchset 2 (id:??) landed as
https://crrev.com/d603dc5c42e8031796c1c2873928ca98cdd24dc2
Cr-Commit-Position: refs/heads/master@{#31432}