Make a pre-read A/B field-trial

chrome/browser/browser_main_win.cc

Index: chrome/browser/browser_main_win.cc
===================================================================
--- chrome/browser/browser_main_win.cc	(revision 95298)
+++ chrome/browser/browser_main_win.cc	(working copy)
@@ -17,6 +17,7 @@
 #include "base/path_service.h"
 #include "base/scoped_native_library.h"
 #include "base/utf_string_conversions.h"
+#include "base/win/registry.h"  // For PreReadExperiment field-trial.
 #include "base/win/windows_version.h"
 #include "base/win/wrapped_window_proc.h"
 #include "chrome/browser/browser_util_win.h"
@@ -96,6 +97,17 @@
   // Record the time to present in a histogram.
   UMA_HISTOGRAM_MEDIUM_TIMES("Startup.BrowserMessageLoopStartTime",
                              elapsed_from_startup);
+
+  // Create a sub-histogram based on the PreReadExperiment coin-toss.
+  base::win::RegKey key(HKEY_CURRENT_USER, L"Software\\Google\\ChromeFrame",

[Sigurður Ásgeirsson, 2011/08/04 13:35:32]

ditto: is there a good common place to put this reading? 

[chrisha, 2011/08/04 14:48:40]

I moved the common functionality to a function declared in browser_main_win.h.

+                        KEY_QUERY_VALUE);
+  DWORD pre_read = 1;
+  const char* uma_name = "Startup.BrowserMessageLoopStartTime_PreReadEnabled";
+  if (key.Valid())
+    key.ReadValueDW(L"PreReadExperiment", &pre_read);
+  if (pre_read == 0)
+    uma_name = "Startup.BrowserMessageLoopStartTime_PreReadDisabled";

[jar (doing other things), 2011/08/03 21:24:34]

personal preference nit: Same comment about refactoring as made in
browser_main.cc to avoid retyping the bulk of the histogram name.

(p.s., typos in these names have been historically bothersome... and sadly
common)

[chrisha, 2011/08/04 14:48:40]

Done.

+  UMA_HISTOGRAM_MEDIUM_TIMES(uma_name, elapsed_from_startup);
 }
 
 int AskForUninstallConfirmation() {