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

ios/web/web_state/web_state.mm

 // Copyright 2014 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.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 WebState::CreateParams::CreateParams(web::BrowserState* browser_state)
     : browser_state(browser_state), created_with_opener(false) {}
 
 WebState::CreateParams::~CreateParams() {}
 
 WebState::OpenURLParams::OpenURLParams(const GURL& url,
                                        const Referrer& referrer,
                                        WindowOpenDisposition disposition,
                                        ui::PageTransition transition,
                                        bool is_renderer_initiated)
     : url(url),
       referrer(referrer),
       disposition(disposition),
       transition(transition),
       is_renderer_initiated(is_renderer_initiated) {}
 
 WebState::OpenURLParams::~OpenURLParams() {}
 
 }  // namespace web