R1:70c864914210

Use phutil_json_decode instead of json_decode

Summary: Generally, `phutil_json_decode` should be preferred over `json_decode`.

Test Plan: Eyellballed.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12680