Mark operator() as const for VS 2015 compat.

device/core/device_monitor_win.cc

 // 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 "device/core/device_monitor_win.h"
 
 #include <dbt.h>
 #include <map>
 #include <windows.h>
 
 #include "base/at_exit.h"
 #include "base/bind.h"
 #include "base/strings/string_util.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/win/message_window.h"
 
 namespace device {
 
 class DeviceMonitorMessageWindow;
 
 namespace {
 
 const wchar_t kWindowClassName[] = L"DeviceMonitorMessageWindow";
 DeviceMonitorMessageWindow* g_message_window;
 
 // Provides basic comparability for GUIDs so that they can be used as keys to an
 // STL map.
 struct CompareGUID {
-  bool operator()(const GUID& a, const GUID& b) {
+  bool operator()(const GUID& a, const GUID& b) const {
     return memcmp(&a, &b, sizeof a) < 0;
   }
 };
 }
 
 // This singleton class manages a shared message window for all registered
 // device notification observers. It vends one instance of DeviceManagerWin for
 // each unique GUID it sees.
 class DeviceMonitorMessageWindow {
  public:
@@ skipping 154 matching lines @@
                     OnDeviceAdded(class_guid, device_path));
 }
 
 void DeviceMonitorWin::NotifyDeviceRemoved(const GUID& class_guid,
                                            const std::string& device_path) {
   FOR_EACH_OBSERVER(Observer, observer_list_,
                     OnDeviceRemoved(class_guid, device_path));
 }
 
 }  // namespace device