Move Singleton and related structs to namespace base

ios/web/browser_url_rewriter_impl.cc

 // 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.
 
 #include "ios/web/browser_url_rewriter_impl.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "ios/web/public/browser_state.h"
 #include "ios/web/public/web_client.h"
@@ skipping 34 matching lines @@
   bool rewritten = false;
   for (URLRewriter rewriter : rewriters) {
     if ((rewritten = rewriter(url, browser_state)))
       break;
   }
   return rewritten;
 }
 
 // static
 BrowserURLRewriterImpl* BrowserURLRewriterImpl::GetInstance() {
-  return Singleton<BrowserURLRewriterImpl>::get();
+  return base::Singleton<BrowserURLRewriterImpl>::get();
 }
 
 BrowserURLRewriterImpl::BrowserURLRewriterImpl() {
   web::WebClient* web_client = web::GetWebClient();
   if (web_client)
     web_client->PostBrowserURLRewriterCreation(this);
 
   // view-source:
   AddURLRewriter(&HandleViewSource);
 }
 
 BrowserURLRewriterImpl::~BrowserURLRewriterImpl() {
 }
 
 void BrowserURLRewriterImpl::AddURLRewriter(URLRewriter rewriter) {
   DCHECK(rewriter);
   url_rewriters_.push_back(rewriter);
 }
 
 bool BrowserURLRewriterImpl::RewriteURLIfNecessary(
     GURL* url,
     BrowserState* browser_state) {
   return BrowserURLRewriter::RewriteURLWithWriters(url, browser_state,
                                                    url_rewriters_);
 }
 
 }  // namespace web