Mark operator() as const for VS 2015 compat.

device/core/device_monitor_win.cc

Index: device/core/device_monitor_win.cc
diff --git a/device/core/device_monitor_win.cc b/device/core/device_monitor_win.cc
index cf2d1f72e0b87ba23ad5635c99183836aac6c1ee..3449c82350e658b5b93379141946f63a8ce993cc 100644
--- a/device/core/device_monitor_win.cc
+++ b/device/core/device_monitor_win.cc
@@ -26,7 +26,7 @@ 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;
   }
 };