[ObjC ARC] Converts ios/web:web to ARC.

ios/web/web_state/web_state_policy_decider.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.
 
 #import "ios/web/public/web_state/web_state_policy_decider.h"
 
 #import "ios/web/public/web_state/web_state.h"
 #import "ios/web/web_state/web_state_impl.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 WebStatePolicyDecider::WebStatePolicyDecider(WebState* web_state)
     : web_state_(web_state) {
   DCHECK(web_state_);
   web_state_->AddPolicyDecider(this);
 }
 
 WebStatePolicyDecider::~WebStatePolicyDecider() {
   if (web_state_) {
     web_state_->RemovePolicyDecider(this);
   }
 }
 
 bool WebStatePolicyDecider::ShouldAllowRequest(NSURLRequest* request) {
   return true;
 }
 
 bool WebStatePolicyDecider::ShouldAllowResponse(NSURLResponse* response) {
   return true;
 }
 
 void WebStatePolicyDecider::ResetWebState() {
   web_state_->RemovePolicyDecider(this);
   web_state_ = nullptr;
 }
 
 }  // namespace web