Add base/parse_number.h to generalize number parsing done in //net.

net/base/parse_number.h

+// Copyright 2016 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 NET_BASE_PARSE_NUMBER_H_
+#define NET_BASE_PARSE_NUMBER_H_
+
+#include "base/compiler_specific.h"
+#include "base/strings/string_piece.h"
+#include "net/base/net_export.h"
+
+// This file contains utility functions for parsing numbers, in the context of
+// network protocols.
+//
+// Q: Doesn't //base already provide these in string_number_conversions.h, with
+//    functions like base::StringToInt()?
+//
+// A: Yes, and those functions are used under the hood by these
+//    implementations.
+//
+//    However using the number parsing functions from //base directly in network
+//    code can lead to subtle bugs, as the //base versions  are more permissive.
+//    For instance "+99" is successfully parsed by base::StringToInt().
+//
+//    However in the majority of places in //net, a leading plus on a number
+//    should be considered invalid. For instance when parsing a host:port pair
+//    you wouldn't want to recognize "foo:+99" as having a port of 99. The same
+//    issue applies when parsing a content-length header.
+//
+//    To reduce the risk of such problems, use of these functions over the
+//    //base versions.
+
+class GURL;
+
+namespace net {
+
+//  Parses a string representing a decimal number to an |int|. Returns true on
+//  success, and fills |*output| with the result. Note that  |*output| is not
+//  modified on failure.
+//
+//  Recognized inputs take the form:
+//    1*DIGIT
+//
+//  Where DIGIT is an ASCII number in the range '0' - '9'
+//
+//  Note that:
+//   * Parsing is locale independent
+//   * Leading zeros are allowed (numbers needn't be in minimal encoding)
+//   * Inputs that would overflow the output are rejected.
+//   * Only accepts integers
+//
+//  Examples of recognized inputs are:
+//    "13"
+//    "0"
+//    "00013"
+//
+//  Examples of rejected inputs are:
+//    "  13"
+//    "-13"
+//    "+13"
+//    "0x15"
+//    "13.3"
+NET_EXPORT bool ParseNonNegativeDecimalInt(const base::StringPiece& input,
+                                           int* output) WARN_UNUSED_RESULT;
+
+}  // namespace net
+
+#endif  // NET_BASE_PARSE_NUMBER_H_