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

net/base/parse_number.cc

+// 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.
+
+#include "net/base/parse_number.h"
+
+#include "base/strings/string_number_conversions.h"
+
+namespace net {
+
+bool ParseNonNegativeDecimalInt(const base::StringPiece& input, int* output) {
+  if (input.empty() || input[0] == '+' || input[0] == '-')

[mmenke, 2016/03/23 19:32:28]

Can we just check if it's not between 0 and 9?  Same thing, given StringToInts
current implementation, and likely all future ones, but I'm paranoid, and just
checking those two characters doesn't save us anything.

[eroman, 2016/03/23 19:50:30]

Done.

(I had same intuition, but somehow rejected it).

An even more paranoid approach is that taken by websockets:

 24 bool GetWindowBits(const std::string& value, int* window_bits) {
 25   return !value.empty() && value[0] != '0' &&
 26          value.find_first_not_of("0123456789") == std::string::npos &&
 27          base::StringToInt(value, window_bits);
 28 }

Which checks for invalid characters throughout.
I think sufficient tests with checking of first character is sufficient though.

+    return false;
+
+  int result;
+  if (!base::StringToInt(input, &result))
+    return false;
+
+  *output = result;
+  return true;
+}
+
+}  // namespace net