Keep WKWebView control group users in the control group

ios/chrome/browser/pref_names.cc

Index: ios/chrome/browser/pref_names.cc
diff --git a/ios/chrome/browser/pref_names.cc b/ios/chrome/browser/pref_names.cc
index 1fec0acef67dbf42a8664c6247bf794fa8bad625..9a204884023d1fbdf5cef9c5794990c1165c3b05 100644
--- a/ios/chrome/browser/pref_names.cc
+++ b/ios/chrome/browser/pref_names.cc
@@ -70,6 +70,10 @@ const char kIosHandoffToOtherDevices[] = "ios.handoff_to_other_devices";
 const char kLastSessionExitedCleanly[] =
     "ios.user_experience_metrics.last_session_exited_cleanly";
 
+// True if the previous session was selected into the WKWebView control group.
+const char kLastSessionUsedWKWebViewControlGroup[] =
+    "ios.wkwebview_trial.was_control";
+
 // Preference that hold a boolean indicating whether metrics reporting should
 // be limited to wifi (when enabled).
 const char kMetricsReportingWifiOnly[] =