Bug 1499590 - Add periodic-file-updates task to .cron.yml. r=dustin

Added configuration for periodic-file-updates task to comm's .cron.yml.

This cron task mirrors one found in mozilla- repositories, and was part of
comm-'s buildbot configuration that needed to be moved to Taskcluster.

Differential Revision: https://phabricator.services.mozilla.com/D9859

.cron.yml

@@ -53,3 +53,25 @@ jobs:
       run-on-projects:
           - comm-central
       when: []  # never (hook only)
+
+    - name: periodic-update
+      job:
+          type: decision-task
+          treeherder-symbol: Nfile
+          target-tasks-method: file_update
+      run-on-projects:
+          - comm-central
+          - comm-beta
+          - comm-esr60
+      when:
+          by-project:
+              # No default branch
+              comm-central:
+                  - {weekday: 'Monday', hour: 10, minute: 0}
+                  - {weekday: 'Thursday', hour: 10, minute: 0}
+              comm-beta:
+                  - {weekday: 'Monday', hour: 10, minute: 0}
+                  - {weekday: 'Thursday', hour: 10, minute: 0}
+              comm-esr60:
+                  - {weekday: 'Monday', hour: 10, minute: 0}
+                  - {weekday: 'Thursday', hour: 10, minute: 0}