Control zero suggest with a pref, not a switch.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index de361fdaf96961aa929d21f1e73ae64e356079b7..2c3e1540da77c2d41b5881021aa79e52e8a6ddab 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -468,6 +468,10 @@ const char kInstantConfirmDialogShown[] = "instant.confirm_dialog_shown";
 // Boolean pref indicating if instant is enabled.
 const char kInstantEnabled[] = "instant.enabled";
 
+// Prefix URL for the (experimental) ZeroSuggest provider.
+const char kExperimentalZeroSuggestUrlPrefix[] =
+    "instant.experimental_zero_suggest_url_prefix";
+
 // Used to migrate preferences from local state to user preferences to
 // enable multiple profiles.
 // BITMASK with possible values (see browser_prefs.cc for enum):