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)
@@ -90,12 +90,9 @@
   ::GetProcessTimes(::GetCurrentProcess(), &creation_time, &ignore, &ignore,
       &ignore);
 
-  base::TimeDelta elapsed_from_startup =
-      base::Time::Now() - base::Time::FromFileTime(creation_time);
-
-  // Record the time to present in a histogram.
-  UMA_HISTOGRAM_MEDIUM_TIMES("Startup.BrowserMessageLoopStartTime",
-                             elapsed_from_startup);
+  RecordPreReadExperimentTime(
+      "Startup.BrowserMessageLoopStartTime",
+      base::Time::Now() - base::Time::FromFileTime(creation_time));
 }
 
 int AskForUninstallConfirmation() {
@@ -276,6 +273,31 @@
   return false;
 }
 
+// This code is specific to the Windows-only PreReadExperiment field-trial.
+void RecordPreReadExperimentTime(const char* name, base::TimeDelta time) {
+  DCHECK(name != NULL);
+
+  static const char kEnvVar[] = "CHROME_PRE_READ_EXPERIMENT";
+  static const base::TimeDelta kMin(base::TimeDelta::FromMilliseconds(1));
+  static const base::TimeDelta kMax(base::TimeDelta::FromHours(1));
+  static const size_t kBuckets(100);
+
+  UMA_HISTOGRAM_CUSTOM_TIMES(name, time, kMin, kMax, kBuckets);
+
+  scoped_ptr<base::Environment> env(base::Environment::Create());
+  DCHECK(env.get() != NULL);
+
+  // Only record the sub-histogram result if the experiment is running
+  // (environment variable is set, and valid).
+  std::string pre_read;
+  if (env->GetVar(kEnvVar, &pre_read) && (pre_read == "0" || pre_read == "1")) {
+    std::string uma_name(name);
+    uma_name += "_PreRead";
+    uma_name += pre_read == "1" ? "Enabled" : "Disabled";
+    UMA_HISTOGRAM_CUSTOM_TIMES(uma_name.c_str(), time, kMin, kMax, kBuckets);
+  }
+}
+
 // BrowserMainPartsWin ---------------------------------------------------------
 
 class BrowserMainPartsWin : public BrowserMainParts {