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

chrome/browser/search_engines/search_provider_install_data.cc

Index: chrome/browser/search_engines/search_provider_install_data.cc
diff --git a/chrome/browser/search_engines/search_provider_install_data.cc b/chrome/browser/search_engines/search_provider_install_data.cc
index 4f9994ecd1ed71dc63e60ff29a7d7e21ef2ae542..225f05933e848fc9adf69654e6b3ea387c2155c0 100644
--- a/chrome/browser/search_engines/search_provider_install_data.cc
+++ b/chrome/browser/search_engines/search_provider_install_data.cc
@@ -138,7 +138,7 @@ void GoogleURLObserver::Observe(int type,
     BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
         NewRunnableMethod(change_notifier_.get(),
                           &GoogleURLChangeNotifier::OnChange,
-                          UIThreadSearchTermsData().GoogleBaseURLValue()));
+                          UIThreadSearchTermsData(NULL).GoogleBaseURLValue()));
   } else {
     // This must be the death notification.
     delete this;
@@ -166,7 +166,7 @@ SearchProviderInstallData::SearchProviderInstallData(
     const NotificationSource& ui_death_source)
     : web_service_(web_service),
       load_handle_(0),
-      google_base_url_(UIThreadSearchTermsData().GoogleBaseURLValue()) {
+      google_base_url_(UIThreadSearchTermsData(NULL).GoogleBaseURLValue()) {
   // GoogleURLObserver is responsible for killing itself when
   // the given notification occurs.
   new GoogleURLObserver(new GoogleURLChangeNotifier(AsWeakPtr()),