[net] Add AsyncDns field trial.

chrome/browser/net/async_dns_field_trial.cc

+// Copyright (c) 2012 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 "chrome/browser/net/async_dns_field_trial.h"
+
+#include "base/metrics/field_trial.h"
+#include "build/build_config.h"
+#include "chrome/common/chrome_version_info.h"
+
+namespace chrome_browser_net {
+
+bool ConfigureAsyncDnsFieldTrial() {
+#if defined(OS_ANDROID) || defined(OS_IOS) || defined(OS_CHROMEOS)
+  // There is no DnsConfigService on those platforms so disable the field trial.
+  return false;
+#endif
+  const base::FieldTrial::Probability kAsyncDnsDivisor = 100;
+  base::FieldTrial::Probability enabled_probability = 0;
+
+  if (chrome::VersionInfo::GetChannel() <= chrome::VersionInfo::CHANNEL_DEV)
+    enabled_probability = 50;
+
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "AsyncDns", kAsyncDnsDivisor, "disabled", 2012, 9, 30, NULL));
+
+  int enabled_group = trial->AppendGroup("enabled", enabled_probability);
+  return trial->group() == enabled_group;
+}
+
+}  // namespace chrome_browser_net