Don't cast SpecialPageFactory::$list to an object

Callers insisde SpecialPageFactory using it as an object updated.

All other callers in extensions et al iterate over the array.

Bug: 67596
Change-Id: Ic917c7d87d5b316b4a2b2bde5932325ff67cf372
Merge "Don't cast SpecialPageFactory::$list to an object"