Use container::back() and container::pop_back().

ios/web/net/request_tracker_impl.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.
 
 #include "ios/web/net/request_tracker_impl.h"
 
 #include <pthread.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 794 matching lines @@
                      estimate));
     }
   }
 }
 
 void RequestTrackerImpl::SSLNotify() {
   DCHECK_CURRENTLY_ON(web::WebThread::IO);
   if (is_closing_)
     return;
 
-  if (!counts_.size())
+  if (counts_.empty())
     return;  // Nothing yet to notify.
 
   if (!page_url_.SchemeIsCryptographic())
     return;
 
   const GURL page_origin = page_url_.GetOrigin();
   ScopedVector<TrackerCounts>::iterator it;
   for (it = counts_.begin(); it != counts_.end(); ++it) {
     if (!(*it)->ssl_info.is_valid())
       continue;  // No SSL info at this point in time on this tracker.
@@ skipping 479 matching lines @@
         }));
   }
 }
 
 void RequestTrackerImpl::SetCertificatePolicyCacheForTest(
     web::CertificatePolicyCache* cache) {
   policy_cache_ = cache;
 }
 
 }  // namespace web