Add a URL param to indicate group selection in Instant field trial.

chrome/browser/instant/instant_field_trial.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_field_trial.h"
 
 #include "base/metrics/field_trial.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/common/pref_names.h"
@@ skipping 20 matching lines @@
   trial->UseOneTimeRandomization();
 
   g_control_group_id_1 = trial->AppendGroup("InstantControl1", 450);  // 45%
   g_control_group_id_2 = trial->AppendGroup("InstantControl2", 450);  // 45%
   g_experiment_group_id_1 = trial->AppendGroup("InstantExperiment1", 50);  // 5%
   g_experiment_group_id_2 = trial->AppendGroup("InstantExperiment2", 50);  // 5%
 }
 
 // static
 InstantFieldTrial::Group InstantFieldTrial::GetGroup(Profile* profile) {
-  if (profile->IsOffTheRecord())
+  if (!profile || profile->IsOffTheRecord())
     return INACTIVE;
 
   const PrefService* prefs = profile->GetPrefs();
   if (!prefs ||
       !prefs->GetBoolean(prefs::kSearchSuggestEnabled) ||
       prefs->GetBoolean(prefs::kInstantEnabledOnce) ||
       prefs->IsManagedPreference(prefs::kInstantEnabled)) {
     return INACTIVE;
   }
 
@@ skipping 22 matching lines @@
   switch (GetGroup(profile)) {
     case INACTIVE:    return "InstantInactive";
     case CONTROL1:    return "InstantControl1";
     case CONTROL2:    return "InstantControl2";
     case EXPERIMENT1: return "InstantExperiment1";
     case EXPERIMENT2: return "InstantExperiment2";
   }
   NOTREACHED();
   return "InstantUnknown";
 }
+
+// static
+std::string InstantFieldTrial::GetGroupAsUrlParam(Profile* profile) {
+  switch (GetGroup(profile)) {
+    case INACTIVE:    return std::string();
+    case CONTROL1:    return "ix=c1&";
+    case CONTROL2:    return "ix=c2&";
+    case EXPERIMENT1: return "ix=e1&";
+    case EXPERIMENT2: return "ix=e2&";
+  }
+  NOTREACHED();
+  return std::string();
+}