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] > '9' || input[0] < '0')
+    return false;
+
+  int result;
+  if (!base::StringToInt(input, &result))
+    return false;
+
+  *output = result;
+  return true;
+}
+
+}  // namespace net