[ObjC ARC] Converts the rest of ios/web to ARC.

ios/web/web_state/web_state_delegate.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.
 
 #import "ios/web/public/web_state/web_state_delegate.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 web {
 
 WebStateDelegate::WebStateDelegate() {}
 
 WebStateDelegate::~WebStateDelegate() {
   while (!attached_states_.empty()) {
     WebState* web_state = *attached_states_.begin();
     web_state->SetDelegate(nullptr);
   }
   DCHECK(attached_states_.empty());
@@ skipping 14 matching lines @@
   DCHECK(attached_states_.find(source) == attached_states_.end());
   attached_states_.insert(source);
 }
 
 void WebStateDelegate::Detach(WebState* source) {
   DCHECK(attached_states_.find(source) != attached_states_.end());
   attached_states_.erase(source);
 }
 
 }  // web