Add histogram to track prerender sessions

chrome/browser/browser_main.h

 // Copyright (c) 2010 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.
 
 #ifndef CHROME_BROWSER_BROWSER_MAIN_H_
 #define CHROME_BROWSER_BROWSER_MAIN_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/metrics/field_trial.h"
@@ skipping 100 matching lines @@
 
   // A/B test for determining a value for unused socket timeout.
   void SocketTimeoutFieldTrial();
 
   // A/B test for the maximum number of connections per proxy server.
   void ProxyConnectionsFieldTrial();
 
   // A/B test for spdy when --use-spdy not set.
   void SpdyFieldTrial();
 
-  // A/B test for prefetching with --(enable|disable)-prefetch not set.
-  void PrefetchFieldTrial();
+  // A/B test for prefetching with --(enable|disable)-prefetch not set,
+  // some histograms tracking prerender sessions.
+  void PrefetchAndPrerenderFieldTrial();
 
   // A/B test for automatically establishing a backup TCP connection when a
   // specified timeout value is reached.
   void ConnectBackupJobsFieldTrial();
 
   // Used to initialize NSPR where appropriate.
   virtual void InitializeSSL() = 0;
 
   // Methods for |MainMessageLoopStart()| --------------------------------------
 
@@ skipping 33 matching lines @@
 // sending crash reports.  The presence of a Breakpad handler (after
 // attempting to initialize crash reporting) and the presence of a debugger
 // are registered with the UMA metrics service.
 void RecordBreakpadStatusUMA(MetricsService* metrics);
 
 // Displays a warning message if some minimum level of OS support is not
 // present on the current platform.
 void WarnAboutMinimumSystemRequirements();
 
 #endif  // CHROME_BROWSER_BROWSER_MAIN_H_