Use an experiment value to set the instant_extended.enabled pref default.

chrome/browser/ui/browser_instant_controller.cc

 // Copyright 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/ui/browser_instant_controller.h"
 
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/themes/theme_service.h"
@@ skipping 54 matching lines @@
 }
 
 bool BrowserInstantController::IsInstantEnabled(Profile* profile) {
   return profile && !profile->IsOffTheRecord() && profile->GetPrefs() &&
          profile->GetPrefs()->GetBoolean(GetInstantPrefName(profile));
 }
 
 void BrowserInstantController::RegisterUserPrefs(PrefServiceSyncable* prefs) {
   prefs->RegisterBooleanPref(prefs::kInstantConfirmDialogShown, false,
                              PrefServiceSyncable::SYNCABLE_PREF);
-  prefs->RegisterBooleanPref(prefs::kInstantExtendedEnabled, true,
+  prefs->RegisterBooleanPref(prefs::kInstantEnabled, false,
                              PrefServiceSyncable::SYNCABLE_PREF);
-  prefs->RegisterBooleanPref(prefs::kInstantEnabled, false,
+
+  search::InstantExtendedDefault instant_extended_default_setting =
+      search::GetInstantExtendedDefaultSetting();
+
+  bool instant_extended_value = true;

[dhollowa, 2013/01/31 21:47:19]

How about using a switch here with no "default:" case?  Then you'll get compiler
support if the IED enum ever changes.

[robertshield, 2013/01/31 22:25:51]

Done.

+  if (instant_extended_default_setting == search::INSTANT_FORCE_OFF)
+    instant_extended_value = false;
+  else if (instant_extended_default_setting == search::INSTANT_USE_EXISTING)
+    instant_extended_value = prefs->GetBoolean(prefs::kInstantEnabled);
+
+  prefs->RegisterBooleanPref(prefs::kInstantExtendedEnabled,
+                             instant_extended_value,
                              PrefServiceSyncable::SYNCABLE_PREF);
 }
 
 bool BrowserInstantController::OpenInstant(WindowOpenDisposition disposition) {
   // Unsupported dispositions.
   if (disposition == NEW_BACKGROUND_TAB || disposition == NEW_WINDOW)
     return false;
 
   // The omnibox currently doesn't use other dispositions, so we don't attempt
   // to handle them. If you hit this DCHECK file a bug and I'll (sky) add
@@ skipping 208 matching lines @@
   // part of the image overlay should draw, 'cos the origin is top-left.
   if (!browser_->search_model()->mode().is_ntp() ||
       theme_info_.theme_id.empty() ||
       theme_info_.image_vertical_alignment == THEME_BKGRND_IMAGE_ALIGN_TOP) {
     return;
   }
   instant_.ThemeAreaHeightChanged(theme_area_height_);
 }
 
 }  // namespace chrome