Add histogram to track prerender sessions

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 1563 matching lines @@
     FilePath path =
         parsed_command_line.GetSwitchValuePath(switches::kDebugPrint);
     printing::PrintedDocument::set_debug_dump_path(path);
   }
 #endif
 
   HandleTestParameters(parsed_command_line);
   RecordBreakpadStatusUMA(metrics);
   about_flags::RecordUMAStatistics(user_prefs);
 
+  UMA_HISTOGRAM_ENUMERATION(

[cbentzel, 2011/01/23 12:32:47]

Could you put this in PrefetchFieldTrial so it's more tied with the other
prerender/prefetch?

This is OK as well since near other UMA calls. 

+      "Prerender.Sessions",
+      parsed_command_line.HasSwitch(switches::kEnablePagePrerender), 2);

[cbentzel, 2011/01/23 12:32:47]

Eventually I think this histogram will want more buckets such as off, manually
disabled, manually enabled, trial-enabled.

[jar (doing other things), 2011/01/23 20:15:53]

I think HasSwitch returns a boolean, but the HISTOGRAM is expected an integer
sample.  I'd like to see a more explicit selection of 0 vs 1.

[gavinp, 2011/01/24 06:01:46]

OK, I've now created an enumeration (which I put in the prerendering directory),
and I use that to seed the histogram.  That should also allow expansion of the
types in this histogram for different session types.  Chris suggested we'd want
this if we don't use the UMA field trial mechanism, and there might well be
other reasons.

+
   // Stat the directory with the inspector's files so that we can know if we
   // should display the entry in the context menu or not.
   browser_process->CheckForInspectorFiles();
 
 #if defined(OS_CHROMEOS)
   metrics->StartExternalMetrics();
 #endif
 
   // Initialize extension event routers. Note that on Chrome OS, this will
   // not succeed if the user has not logged in yet, in which case the
@@ skipping 155 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("BrowserMain", 0, 0);
   return result_code;
 }