Factor out common error-code conversion code for PlatformFileErrorToNetError.

net/base/net_errors.h

Index: net/base/net_errors.h
diff --git a/net/base/net_errors.h b/net/base/net_errors.h
index 12344e77609e2760ecff6ed754fdb7fc8dababb3..1adcdda91a027739f37bfec0ba84493fa1eb6663 100644
--- a/net/base/net_errors.h
+++ b/net/base/net_errors.h
@@ -8,6 +8,7 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/platform_file.h"
 #include "net/base/net_export.h"
 
 namespace net {
@@ -50,6 +51,10 @@ NET_EXPORT Error MapSystemError(int os_error);
 // error code that is not followed immediately by a valid error code.
 std::vector<int> GetAllErrorCodesForUma();
 
+// A convenient function to translate platform file error to net error code.
+NET_EXPORT Error PlatformFileErrorToNetError(
+    base::PlatformFileError file_error);
+
 }  // namespace net
 
 #endif  // NET_BASE_NET_ERRORS_H__