All together now

content/public/common/desktop_media_id.cc

 // Copyright 2013 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 "content/public/common/desktop_media_id.h"
 
 namespace content {
 
 // static
 DesktopMediaID DesktopMediaID::Parse(const std::string& str) {
   if (str == "screen")
     return DesktopMediaID(TYPE_SCREEN, 0);
 
   std::string window_prefix("window:");
   if (StartsWithASCII(str, window_prefix, true)) {
     int64 id;
     if (!base::StringToInt64(str.substr(window_prefix.size()), &id))
       return DesktopMediaID(TYPE_NONE, 0);
     return DesktopMediaID(TYPE_WINDOW, id);
   }
 
   return DesktopMediaID(TYPE_NONE, 0);
 }
 
-std::string DesktopMediaID::ToString() {
+std::string DesktopMediaID::ToString() const {
   switch (type) {
     case TYPE_NONE:
       NOTREACHED();
       return std::string();
 
     case TYPE_SCREEN:
       return "screen";
 
     case TYPE_WINDOW:
       return "window:" + base::Int64ToString(id);
   }
   NOTREACHED();
   return std::string();
 }
 
 }  // namespace content