Fix OAuth Facebook Phame property access

Summary: Ref T1536. This is missing a call.

Test Plan: Viewed a public blog with Facebook comments.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6288

src/applications/auth/provider/PhabricatorAuthProviderOAuthFacebook.php

@@ -121,7 +121,7 @@ final class PhabricatorAuthProviderOAuthFacebook
       return null;
     }
 
-    return $fb_provider->getProperty(
+    return $fb_provider->getProviderConfig()->getProperty(
       PhabricatorAuthProviderOAuth::PROPERTY_APP_ID);
   }