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

ash/accelerators/accelerator_controller.cc

 // Copyright (c) 2012 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 "ash/accelerators/accelerator_controller.h"
 
 #include <algorithm>
 #include <cmath>
 #include <string>
 #include <utility>
@@ skipping 126 matching lines @@
   // The plus sign surrounded by the word joiner to guarantee an non-breaking
   // shortcut.
   const base::string16 non_breaking_plus =
       base::UTF8ToUTF16("\xe2\x81\xa0+\xe2\x81\xa0");
   shortcut_text->clear();
   for (size_t i = 0; i < keys.size() - 1; ++i) {
     *shortcut_text += keys[i];
     *shortcut_text += non_breaking_plus;
   }
 
-  *shortcut_text += keys[keys.size() - 1];
+  *shortcut_text += keys.back();
 }
 
 // Gets the notification message after it formats it in such a way that there
 // are no line breaks in the middle of the shortcut texts.
 base::string16 GetNotificationText(int message_id,
                                    int old_shortcut_id,
                                    int new_shortcut_id) {
   base::string16 old_shortcut = l10n_util::GetStringUTF16(old_shortcut_id);
   base::string16 new_shortcut = l10n_util::GetStringUTF16(new_shortcut_id);
   EnsureNoWordBreaks(&old_shortcut);
@@ skipping 1250 matching lines @@
   if (wm_shell->mru_window_tracker()->BuildMruWindowList().empty() &&
       actions_needing_window_.find(action) != actions_needing_window_.end()) {
     wm_shell->GetAccessibilityDelegate()->TriggerAccessibilityAlert(
         A11Y_ALERT_WINDOW_NEEDED);
     return RESTRICTION_PREVENT_PROCESSING_AND_PROPAGATION;
   }
   return RESTRICTION_NONE;
 }
 
 }  // namespace ash