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 38ee088d11e03d1a6519d644b2eccdb447d3d3b6..65274d96782336b6028cae90f7e9a7634c47c5fd 100644
--- a/chrome/browser/instant/instant_field_trial.cc
+++ b/chrome/browser/instant/instant_field_trial.cc
@@ -38,7 +38,7 @@ void InstantFieldTrial::Activate() {
 
 // static
 InstantFieldTrial::Group InstantFieldTrial::GetGroup(Profile* profile) {
-  if (profile->IsOffTheRecord())
+  if (!profile || profile->IsOffTheRecord())
     return INACTIVE;
 
   const PrefService* prefs = profile->GetPrefs();
@@ -81,3 +81,16 @@ std::string InstantFieldTrial::GetGroupName(Profile* profile) {
   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();
+}