[fsp] Convert base::File::Error to string.

base/files/file.h

 // Copyright (c) 2012 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_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
 #if defined(OS_POSIX)
 #include <sys/stat.h>
 #endif
 
+#include <string>
+
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/scoped_file.h"
 #include "base/move.h"
 #include "base/time/time.h"
 
 #if defined(OS_WIN)
 #include "base/win/scoped_handle.h"
 #endif
 
@@ skipping 255 matching lines @@
   Error Unlock();
 
   bool async() const { return async_; }
 
 #if defined(OS_WIN)
   static Error OSErrorToFileError(DWORD last_error);
 #elif defined(OS_POSIX)
   static Error OSErrorToFileError(int saved_errno);
 #endif
 
+  // Converts an error value to a human-readable form. Used for logging.
+  static std::string ErrorToString(Error error);
+
  private:
   void SetPlatformFile(PlatformFile file);
 
 #if defined(OS_WIN)
   win::ScopedHandle file_;
 #elif defined(OS_POSIX)
   ScopedFD file_;
 #endif
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_