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

services/ui/ws/display_manager.cc

 // Copyright 2016 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 "services/ui/ws/display_manager.h"
 
 #include "base/memory/ptr_util.h"
 #include "services/ui/ws/display.h"
 #include "services/ui/ws/display_manager_delegate.h"
 #include "services/ui/ws/event_dispatcher.h"
@@ skipping 43 matching lines @@
     for (const auto& pair : user_display_managers_)
       pair.second->OnWillDestroyDisplay(display);
 
     DCHECK(displays_.count(display));
     displays_.erase(display);
   }
   delete display;
 
   // If we have no more roots left, let the app know so it can terminate.
   // TODO(sky): move to delegate/observer.
-  if (!displays_.size() && !pending_displays_.size())
+  if (displays_.empty() && pending_displays_.empty())
     delegate_->OnNoMoreDisplays();
 }
 
 void DisplayManager::DestroyAllDisplays() {
   while (!pending_displays_.empty())
     DestroyDisplay(*pending_displays_.begin());
   DCHECK(pending_displays_.empty());
 
   while (!displays_.empty())
     DestroyDisplay(*displays_.begin());
@@ skipping 83 matching lines @@
   }
 
   WindowManagerState* current_window_manager_state =
       delegate_->GetWindowManagerStateForUser(active_id);
   if (current_window_manager_state)
     current_window_manager_state->Activate(mouse_location_on_screen);
 }
 
 }  // namespace ws
 }  // namespace ui