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

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index f1a83774becfa65649cf23913363d59db7cd92c9..1913bc7f6c408c8e1e3840da712895ff2fd88259 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -14,6 +14,8 @@
 #include <sys/stat.h>
 #endif
 
+#include <string>
+
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/files/scoped_file.h"
@@ -289,6 +291,9 @@ class BASE_EXPORT File {
   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);