[ARC] Converts parts of ios/web/web_state to ARC.

ios/web/web_state/global_web_state_event_tracker.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/web/web_state/global_web_state_event_tracker.h"
 
 #include <stddef.h>
 
 #include "base/macros.h"
 #include "base/memory/singleton.h"
 #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;
 DEFINE_WEB_STATE_USER_DATA_KEY(WebStateEventForwarder);
 
 // Helper class that forwards events from a single associated WebState instance
 // to the GlobalWebStateEventTracker singleton.
 class WebStateEventForwarder : public WebStateUserData<WebStateEventForwarder>,
                                public WebStateObserver {
  public:
@@ skipping 116 matching lines @@
   FOR_EACH_OBSERVER(GlobalWebStateObserver, observer_list_,
                     PageLoaded(web_state, load_completion_status));
 }
 
 void GlobalWebStateEventTracker::WebStateDestroyed(WebState* web_state) {
   FOR_EACH_OBSERVER(GlobalWebStateObserver, observer_list_,
                     WebStateDestroyed(web_state));
 }
 
 }  // namespace web