Allow access to Array Iterator through the API, in order to simplify
setting up interfaces which use these methods. This applies to
WebIDL interfaces with "length" attributes returning integer types and
a getter taking an unsigned long type.

BUG=
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/1378403004

Cr-Commit-Position: refs/heads/master@{#31152}