Add fuzz testers to //net for some functions with simple parsing APIs.

net/proxy/parse_proxy_list_pac_fuzzer.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 <stddef.h>
+#include <stdint.h>
+
+#include "net/proxy/proxy_list.h"
+
+// Entry point for LibFuzzer.
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  net::ProxyList list;
+  std::string input(data, data + size);
+  list.SetFromPacString(input);
+  return 0;
+}