[ARC] Converts part of ios/web/ui to ARC.

ios/web/web_state/js/crw_js_plugin_placeholder_manager.mm

 // Copyright 2013 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/web_state/js/crw_js_plugin_placeholder_manager.h"
 
 #include "base/logging.h"
 #include "base/strings/sys_string_conversions.h"
 #include "ios/web/public/web_client.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation CRWJSPluginPlaceholderManager
 
 namespace {
 
 // Returns a string with \ and ' escaped, and wrapped in '.
 // This is used instead of GetQuotedJSONString because that will convert
 // UTF-16 to UTF-8, which can cause problems when injecting scripts depending
 // on the page encoding (see crbug.com/302741).
 NSString* EscapedQuotedString(NSString* string) {
   string = [string stringByReplacingOccurrencesOfString:@"\\"
@@ skipping 17 matching lines @@
   DCHECK(web::GetWebClient());
   NSString* pluginNotSupportedText = base::SysUTF16ToNSString(
       web::GetWebClient()->GetPluginNotSupportedText());
   NSString* placeholderCall = [NSString stringWithFormat:
       @"__gCrWeb.plugin.addPluginPlaceholders(%@);",
           EscapedQuotedString(pluginNotSupportedText)];
   return [baseContent stringByAppendingString:placeholderCall];
 }
 
 @end