Converts parts of ios/web to ARC

ios/web/active_state_manager_impl.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/active_state_manager_impl.h"
 
 #include "base/logging.h"
 #include "ios/web/public/browser_state.h"
 #include "ios/web/public/web_thread.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 namespace {
 // The number of ActiveStateManagers that are currently in active state.
 // At most one ActiveStateManager can be active at any given time.
 int g_active_state_manager_active_count = 0;
 }  // namespace
 
 ActiveStateManagerImpl::ActiveStateManagerImpl(BrowserState* browser_state)
     : browser_state_(browser_state), active_(false) {
@@ skipping 36 matching lines @@
   DCHECK_CURRENTLY_ON(WebThread::UI);
   observer_list_.AddObserver(obs);
 }
 
 void ActiveStateManagerImpl::RemoveObserver(ActiveStateManager::Observer* obs) {
   DCHECK_CURRENTLY_ON(WebThread::UI);
   observer_list_.RemoveObserver(obs);
 }
 
 }  // namespace web