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

chrome/browser/instant/instant_field_trial.cc

Index: chrome/browser/instant/instant_field_trial.cc
diff --git a/chrome/browser/instant/instant_field_trial.cc b/chrome/browser/instant/instant_field_trial.cc
index 90016ff3003085d050cc50efbc519781a21256b9..74dbc00d46b61bd2213108b140c4191c6968e068 100644
--- a/chrome/browser/instant/instant_field_trial.cc
+++ b/chrome/browser/instant/instant_field_trial.cc
@@ -58,3 +58,16 @@ std::string InstantFieldTrial::GetGroupName(Profile* profile) {
   NOTREACHED();
   return "InstantUnknown";
 }
+
+// static
+std::string InstantFieldTrial::GetGroupAsUrlParam(Profile* profile) {
+  switch (GetGroup(profile)) {
+    case INACTIVE:    return "";
+    case CONTROL1:    return "ix=c1&";
+    case CONTROL2:    return "ix=c2&";
+    case EXPERIMENT1: return "ix=e1&";
+    case EXPERIMENT2: return "ix=e2&";
+  }
+  NOTREACHED();
+  return "";
+}