Ensure JSProxy correctness for PrototypeIterator uses

Ensure JSProxy correctness for PrototypeIterator uses

This CL fixes an invalid cast in Slow_ArrayConcat (a Proxy on a DICTIONARY_ELEMENTS array's prototype chain).
It also adds some comments and minor drive-by refactorings to other PrototypeIterator use sites.

R=verwaest@chromium.org

Committed: https://crrev.com/48f4cbc7c3503a934f89b7cd2b9e8878795123ba
Cr-Commit-Position: refs/heads/master@{#31738}

[Toon Verwaest, 2015-11-03 09:44:17]

lgtm

[Jakob Kummerow, 2015-11-03 10:18:37]

The CQ bit was checked by jkummerow@chromium.org

[commit-bot: I haz the power, 2015-11-03 10:18:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1402393003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1402393003/20001

[commit-bot: I haz the power, 2015-11-03 10:43:10]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-03 10:43:47]

Patchset 2 (id:??) landed as
https://crrev.com/48f4cbc7c3503a934f89b7cd2b9e8878795123ba
Cr-Commit-Position: refs/heads/master@{#31738}