Set up field trial for precache experiment

chrome/browser/chrome_browser_field_trials_mobile.cc

 // Copyright 2013 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/chrome_browser_field_trials_mobile.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/metrics/field_trial.h"
 #include "base/prefs/pref_service.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/chrome_version_info.h"
 
 namespace chrome {
 
+const char kEnabled[] = "Enabled";
+const char kDisabled[] = "Disabled";
+
 namespace {
 
 // Base function used by all data reduction proxy field trials. A trial is
 // only conducted for installs that are in the "Enabled" group. They are always
 // enabled on DEV and BETA channels, and for STABLE, a percentage will be
 // controlled from the server. Until the percentage is learned from the server,
 // a client-side configuration is used.
 void DataCompressionProxyBaseFieldTrial(
     const char* trial_name,
     const base::FieldTrial::Probability enabled_group_probability,
     const base::FieldTrial::Probability total_probability) {
-  const char kEnabled[] = "Enabled";
-  const char kDisabled[] = "Disabled";
 
   // Find out if this is a stable channel.
   const bool kIsStableChannel =
       chrome::VersionInfo::GetChannel() == chrome::VersionInfo::CHANNEL_STABLE;
 
   // Experiment enabled until Jan 1, 2015. By default, disabled.
   scoped_refptr<base::FieldTrial> trial(
       base::FieldTrialList::FactoryGetFieldTrial(
           trial_name,
           total_probability,
@@ skipping 19 matching lines @@
 
   // Governs the rollout of the _promo_ for the compression proxy
   // independently from the rollout of compression proxy. The enabled
   // percentage is configured to be 10% = 100 / 1000 until overridden by the
   // server. When this trial is "Enabled," users get a promo, whereas
   // otherwise, compression is available without a promo.
   DataCompressionProxyBaseFieldTrial(
       "DataCompressionProxyPromoVisibility", 100, 1000);
 }
 
+// Configures the "Precache" field trial. A trial is only conducted for installs
+// that are in the "Enabled" group. Until the percentage is learned from the
+// server, a client-side configuration is used.
+void PrecacheFieldTrial() {

[Alexei Svitkine (slow), 2014/02/12 15:41:50]

I'm not sure this block of code actually buys you anything, since this will be a
server-driven experiment.

Basically, it's still possible to have a server-driven experiment without this
code being present. The only thing this buys is a configuration when the Finch
config isn't available (e.g. first run after the initial install). But since
this hardcoded config sets it to 100% disabled, that shouldn't be any different
than not having a config.

+  // Experiment enabled until December 31, 2015. By default, disabled.
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "Precache", 1000, kDisabled, 2015, 12, 31,
+          base::FieldTrial::ONE_TIME_RANDOMIZED, NULL));
+
+  const int kEnabledGroup = trial->AppendGroup(kEnabled, 0);
+
+  VLOG(1) << "Precache enabled group id: " << kEnabledGroup
+          << ". Selected group id: " << trial->group();
+}
+
 }  // namespace
 
 void SetupMobileFieldTrials(const CommandLine& parsed_command_line,
                             const base::Time& install_time,
                             PrefService* local_state) {
   DataCompressionProxyFieldTrials();
+  PrecacheFieldTrial();
 }
 
 }  // namespace chrome