[ObjC ARC] Converts ios/chrome/browser/prefs:prefs to ARC.

ios/chrome/browser/prefs/pref_observer_bridge.mm

Index: ios/chrome/browser/prefs/pref_observer_bridge.mm
diff --git a/ios/chrome/browser/prefs/pref_observer_bridge.mm b/ios/chrome/browser/prefs/pref_observer_bridge.mm
index 4ba85bb15ddb835f3d4633f2c42b6fdab49f28c8..61083eb6755b5c8e67b9b66d8a10e573bd303dce 100644
--- a/ios/chrome/browser/prefs/pref_observer_bridge.mm
+++ b/ios/chrome/browser/prefs/pref_observer_bridge.mm
@@ -7,6 +7,10 @@
 #include "base/bind.h"
 #include "components/prefs/pref_change_registrar.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 PrefObserverBridge::PrefObserverBridge(id<PrefObserverDelegate> delegate)
     : delegate_(delegate) {
 }