Fix AmountOfFreeDiskSpace to be able to differentiate an error...

base/sys_info.h

 // Copyright (c) 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_SYS_INFO_H_
 #define BASE_SYS_INFO_H_
 
 #include "base/basictypes.h"
 
 #include <string>
 
 namespace base {
 
 class SysInfo {
  public:
   // Return the number of logical processors/cores on the current machine.
   static int NumberOfProcessors();
   
   // Return the number of bytes of physical memory on the current machine.
   static int64 AmountOfPhysicalMemory();
 
   // Return the number of megabytes of physical memory on the current machine.
   static int AmountOfPhysicalMemoryMB() {
     return static_cast<int>(AmountOfPhysicalMemory() / 1024 / 1024);
   }
 
-  // Return the available disk space in bytes on the volume containing |path|.
+  // Return the available disk space in bytes on the volume containing |path|,
+  // or -1 on failure.
   static int64 AmountOfFreeDiskSpace(const std::wstring& path);
 
 };
 
 }  // namespace base
 
 #endif  // BASE_SYS_INFO_H_