[ObjC ARC] Converts ios/chrome/browser/ui/settings:unit_tests to ARC.

Automatically generated ARCMigrate commit
Notable issues:
* added autorelease pools in tests related to SettingsNavigationControllerTest to fix
 memory issues, specifically early deallocation of SigninManager where the observers
 (SigninObserverBridge of SettingsCollectionViewController) stored in WeakNSObject
 are not deallocated yet (in autorelease pool)
* Removes NS_RETURNS_RETAINED from a method that doesn't really need it

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2806323003
Cr-Commit-Position: refs/heads/master@{#464013}
36 files changed