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

Review URL: https://codereview.chromium.org/1402393003

Cr-Commit-Position: refs/heads/master@{#31738}