Fix `cancdn` vs `canCDN` flag

Summary:
Ref T5884. We migrated to add a `canCDN` flag, but the code looks for a `cancdn` flag.

If this fixes the issue, I'll migrate `cancdn` to `canCDN` in the next diff.

Test Plan: Viewed some files, including old files, and saw the cacheability I expected.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5884

Differential Revision: https://secure.phabricator.com/D10264