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

ios/chrome/browser/ui/contextual_search/contextual_search_web_state_observer.mm

 // Copyright 2016 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/chrome/browser/ui/contextual_search/contextual_search_web_state_observer.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 ContextualSearchWebStateObserver::ContextualSearchWebStateObserver(
     id<ContextualSearchWebStateDelegate> delegate)
     : web::WebStateObserver(), delegate_(delegate), loaded_(false) {}
 
 ContextualSearchWebStateObserver::~ContextualSearchWebStateObserver() {}
 
 void ContextualSearchWebStateObserver::ObserveWebState(
     web::WebState* web_state) {
   loaded_ = false;
   Observe(web_state);
@@ skipping 15 matching lines @@
         pageLoadedWithStatus:load_completion_status];
   }
   loaded_ = true;
 }
 
 void ContextualSearchWebStateObserver::WebStateDestroyed() {
   if ([delegate_ respondsToSelector:@selector(webStateDestroyed:)]) {
     [delegate_ webStateDestroyed:web_state()];
   }
 }