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

chrome/browser/ui/browser_instant_controller.cc

Index: chrome/browser/ui/browser_instant_controller.cc
diff --git a/chrome/browser/ui/browser_instant_controller.cc b/chrome/browser/ui/browser_instant_controller.cc
index 7abe3aa900015a2cc93088f32a243fdf112b7983..555849464c75c076070041f0548edc644c00eff5 100644
--- a/chrome/browser/ui/browser_instant_controller.cc
+++ b/chrome/browser/ui/browser_instant_controller.cc
@@ -72,10 +72,21 @@ bool BrowserInstantController::IsInstantEnabled(Profile* profile) {
 void BrowserInstantController::RegisterUserPrefs(PrefServiceSyncable* prefs) {
   prefs->RegisterBooleanPref(prefs::kInstantConfirmDialogShown, false,
                              PrefServiceSyncable::SYNCABLE_PREF);
-  prefs->RegisterBooleanPref(prefs::kInstantExtendedEnabled, true,
-                             PrefServiceSyncable::SYNCABLE_PREF);
   prefs->RegisterBooleanPref(prefs::kInstantEnabled, false,
                              PrefServiceSyncable::SYNCABLE_PREF);
+
+  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) {