[ARC] Converts part of ios/web/ui to ARC.

ios/web/web_state/global_web_state_event_tracker.mm

Index: ios/web/web_state/global_web_state_event_tracker.mm
diff --git a/ios/web/web_state/global_web_state_event_tracker.mm b/ios/web/web_state/global_web_state_event_tracker.mm
index 9a51bf5f0392717fa1b4896fd579115cf40a9129..267b38c1046fcc41d68e98873f3ee3b635bc5c70 100644
--- a/ios/web/web_state/global_web_state_event_tracker.mm
+++ b/ios/web/web_state/global_web_state_event_tracker.mm
@@ -11,6 +11,10 @@
 #include "ios/web/public/web_state/web_state_observer.h"
 #include "ios/web/public/web_state/web_state_user_data.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 class WebStateEventForwarder;