Make a pre-read A/B field-trial

chrome/browser/browser_main.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/browser_main.h"
 
 #include <algorithm>
 #include <string>
 #include <vector>
 
@@ skipping 2008 matching lines @@
       upgrade_util::SaveLastModifiedTimeOfExe();
 #endif
 
       // Record now as the last successful chrome start.
       GoogleUpdateSettings::SetLastRunTime();
       // Call Recycle() here as late as possible, before going into the loop
       // because Start() will add things to it while creating the main window.
       if (pool)
         pool->Recycle();
 
-      UMA_HISTOGRAM_MEDIUM_TIMES("Startup.BrowserOpenTabs",
-                                 base::TimeTicks::Now() - browser_open_start);
+      RecordPreReadExperimentTime("Startup.BrowserOpenTabs",
+                                  base::TimeTicks::Now() - browser_open_start);
 
       // TODO(mad): Move this call in a proper place on CrOS.
       // http://crosbug.com/17687
 #if !defined(OS_CHROMEOS)
       // If we're running tests (ui_task is non-null), then we don't want to
       // call FetchLanguageListFromTranslateServer
       if (parameters.ui_task == NULL && translate_manager != NULL) {
         // TODO(willchan): Get rid of this after TranslateManager doesn't use
         // the default request context. http://crbug.com/89396.
         // This is necessary to force |default_request_context_| to be
@@ skipping 74 matching lines @@
 #if defined(OS_CHROMEOS)
   // To be precise, logout (browser shutdown) is not yet done, but the
   // remaining work is negligible, hence we say LogoutDone here.
   chromeos::BootTimesLoader::Get()->AddLogoutTimeMarker("LogoutDone",
                                                         false);
   chromeos::BootTimesLoader::Get()->WriteLogoutTimes();
 #endif
   TRACE_EVENT_END_ETW("BrowserMain", 0, 0);
   return result_code;
 }