Move some XDG code from chrome to base.

base/base_paths.h

 // Copyright (c) 2006-2008 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.
 
 #ifndef BASE_BASE_PATHS_H_
 #define BASE_BASE_PATHS_H_
 
 // This file declares path keys for the base module.  These can be used with
 // the PathService to access various special directories and files.
 
 #include "base/basictypes.h"
 #if defined(OS_WIN)
 #include "base/base_paths_win.h"
 #elif defined(OS_MACOSX)
 #include "base/base_paths_mac.h"
 #endif
-#include "base/path_service.h"
 
 namespace base {
 
 enum {
   PATH_START = 0,
 
   DIR_CURRENT,  // current directory
   DIR_EXE,      // directory containing FILE_EXE
   DIR_MODULE,   // directory containing FILE_MODULE
   DIR_TEMP,     // temporary directory
   FILE_EXE,     // Path and filename of the current executable.
   FILE_MODULE,  // Path and filename of the module containing the code for the
                 // PathService (which could differ from FILE_EXE if the
                 // PathService were compiled into a shared object, for example).
   DIR_SOURCE_ROOT,  // Returns the root of the source tree.  This key is useful
                     // for tests that need to locate various resources.  It
                     // should not be used outside of test code.
+#if defined(OS_LINUX)
+  DIR_USER_CACHE,   // Directory where user cache data resides. The Chromium
+                    // browser cache can be a subdirectory of DIR_USER_CACHE.
+                    // This is $XDG_CACHE_HOME on Linux.
+#endif
 
   PATH_END
 };
 
 }  // namespace base
 
 #endif  // BASE_BASE_PATHS_H_