Simplify one "array_keys/range" -> "phutil_is_natural_list()" in "phabricator/"

Summary: Depends on D20213. Simplify this idiom.

Test Plan: Squinted hard; grep array_keys | grep range.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20215