Bug 1535057: Always re-fetch chromium in cron; r=aki

We have a cron job to update the chromium, but the tasks were getting optimized
away. Force the target tasks for the cron job to always run.

Differential Revision: https://phabricator.services.mozilla.com/D23380

--HG--
extra : moz-landing-system : lando

.cron.yml

@@ -116,6 +116,7 @@ jobs:
           type: decision-task
           treeherder-symbol: Chromium
           target-tasks-method: chromium_update
+          optimize-target-tasks: false
       run-on-projects:
           - mozilla-central
       when: