Enable the freeze option for JSHint

Summary:
This option prohibits overwriting prototypes of native objects such as Array, Date and so on.

// jshint freeze:true
Array.prototype.count = function (value) { return 4; };
// -> Warning: Extending prototype of native object: 'Array'.

Test Plan: Linted existing JavaScript files, found no violations.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11439