Restrict Instant field trial to UMA opt-in users.

chrome/browser/instant/instant_controller.cc

 // Copyright (c) 2011 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/instant/instant_controller.h"
 
 #include "base/command_line.h"
 #include "base/message_loop.h"
 #include "base/metrics/histogram.h"
 #include "base/rand_util.h"
@@ skipping 58 matching lines @@
                              PrefService::UNSYNCABLE_PREF);
   prefs->RegisterBooleanPref(prefs::kInstantEnabled,
                              false,
                              PrefService::UNSYNCABLE_PREF);
   prefs->RegisterBooleanPref(prefs::kInstantEnabledOnce,
                              false,
                              PrefService::UNSYNCABLE_PREF);
   prefs->RegisterInt64Pref(prefs::kInstantEnabledTime,
                            false,
                            PrefService::UNSYNCABLE_PREF);
-  prefs->RegisterIntegerPref(prefs::kInstantFieldTrialRandomDraw,
-                             base::RandInt(0, 9999),
-                             PrefService::UNSYNCABLE_PREF);
   PromoCounter::RegisterUserPrefs(prefs, prefs::kInstantPromo);
 }
 
 // static
 void InstantController::RecordMetrics(Profile* profile) {
   if (!IsEnabled(profile))
     return;
 
   PrefService* service = profile->GetPrefs();
   if (service) {
@@ skipping 657 matching lines @@
   if (destroy_factory_.empty()) {
     MessageLoop::current()->PostTask(
         FROM_HERE, destroy_factory_.NewRunnableMethod(
             &InstantController::DestroyLoaders));
   }
 }
 
 void InstantController::DestroyLoaders() {
   loaders_to_destroy_.reset();
 }