[ObjC ARC] Converts ios/web/public/test:test to ARC.

ios/web/public/test/test_redirect_observer.mm

Index: ios/web/public/test/test_redirect_observer.mm
diff --git a/ios/web/public/test/test_redirect_observer.mm b/ios/web/public/test/test_redirect_observer.mm
index d57620728b7ea524dd45341a7b3251b1d1151c0c..c1629113cfec182ed363a52642dd4a06caf5ea4f 100644
--- a/ios/web/public/test/test_redirect_observer.mm
+++ b/ios/web/public/test/test_redirect_observer.mm
@@ -11,6 +11,10 @@
 #include "ios/web/public/web_state/navigation_context.h"
 #import "ios/web/public/web_state/web_state.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // The key under which TestRedirectObservers are stored in a WebState's user
 // data.