[ObjC ARC] Converts ios/chrome/browser/web:web_internal to ARC.

ios/chrome/browser/web/error_page_generator.mm

 // Copyright 2017 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/chrome/browser/web/error_page_generator.h"
 
 #import "base/ios/ns_error_util.h"
 #include "base/logging.h"
-#import "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/values.h"
 #include "components/error_page/common/error_page_params.h"
 #include "components/error_page/common/localized_error.h"
 #include "components/grit/components_resources.h"
 #include "ios/chrome/browser/application_context.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/base/resource/scale_factor.h"
 #include "ui/base/webui/jstemplate_builder.h"
 #include "url/gurl.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation ErrorPageGenerator {
   // Stores the HTML generated from the NSError in the initializer.
-  base::scoped_nsobject<NSString> html_;
+  NSString* _HTML;
 }
 
 - (instancetype)initWithError:(NSError*)error
                        isPost:(BOOL)isPost
                   isIncognito:(BOOL)isIncognito {
   self = [super init];
   if (self) {
     NSString* badURLSpec = error.userInfo[NSURLErrorFailingURLStringErrorKey];
     NSError* originalError = base::ios::GetFinalUnderlyingErrorFromError(error);
     NSString* errorDomain = nil;
@@ skipping 18 matching lines @@
     ui::ScaleFactor scaleFactor =
         ResourceBundle::GetSharedInstance().GetMaxScaleFactor();
 
     const base::StringPiece templateHTML(
         ResourceBundle::GetSharedInstance().GetRawDataResourceForScale(
             IDR_NET_ERROR_HTML, scaleFactor));
     if (templateHTML.empty())
       NOTREACHED() << "unable to load template. ID: " << IDR_NET_ERROR_HTML;
     std::string errorHTML = webui::GetTemplatesHtml(
         templateHTML, &errorStrings, "t" /* IDR_NET_ERROR_HTML root id */);
-    html_.reset([base::SysUTF8ToNSString(errorHTML) retain]);
+    _HTML = base::SysUTF8ToNSString(errorHTML);
   }
   return self;
 }
 
 #pragma mark - HtmlGenerator
 
 - (void)generateHtml:(HtmlCallback)callback {
-  callback(html_.get());
+  callback(_HTML);
 }
 
 @end