Fix a crash when processing an invalid PAC script.

net/data/proxy_resolver_v8_unittest/exception_findproxyforurl_during_resolve.js

+// Copyright (c) 2015 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.
+
+// This PAC script will throw an exception when accessing the "FindProxyForURL"
+// the *second* time it is accessed.
+
+function actualFindProxyForURL(url, host) {
+  return "DIRECT";
+}
+
+var counter = 2;
+
+function FindProxyForURLGetter() {
+  counter--;
+  if (counter <= 0)
+    throw "crash!";
+  return actualFindProxyForURL;
+}
+
+Object.defineProperty(this, "FindProxyForURL", {get: FindProxyForURLGetter});