Make a pre-read A/B field-trial

chrome/browser/browser_main.cc

Index: chrome/browser/browser_main.cc
===================================================================
--- chrome/browser/browser_main.cc	(revision 95298)
+++ chrome/browser/browser_main.cc	(working copy)
@@ -165,6 +165,7 @@
 #include <commctrl.h>
 #include <shellapi.h>
 
+#include "base/environment.h"  // For PreRead experiment.
 #include "base/win/scoped_com_initializer.h"
 #include "base/win/windows_version.h"
 #include "chrome/browser/browser_trial.h"
@@ -227,6 +228,48 @@
 class NetLog;
 }  // namespace net
 
+// This code is specific to the Windows-only PreReadExperiment field-trial.
+static void AddPreReadHistogramTime(const char* name, base::TimeDelta time) {
+  const base::TimeDelta kMin(base::TimeDelta::FromMilliseconds(1));
+  const base::TimeDelta kMax(base::TimeDelta::FromHours(1));
+  static const size_t kBuckets(100);
+
+  // FactoryTimeGet will always return a pointer to the same histogram object,
+  // keyed on its name. There's no need for us to store it explicitly anywhere.
+  base::Histogram* counter = base::Histogram::FactoryTimeGet(
+      name, kMin, kMax, kBuckets, base::Histogram::kUmaTargetedHistogramFlag);
+
+  counter->AddTime(time);
+}
+
+// This code is specific to the Windows-only PreReadExperiment field-trial.
+void RecordPreReadExperimentTime(const char* name, base::TimeDelta time) {
+  DCHECK(name != NULL);
+
+  // This gets called with different histogram names, so we don't want to use
+  // the UMA_HISTOGRAM_CUSTOM_TIMES macro--it uses a static variable, and the
+  // first call wins.
+  AddPreReadHistogramTime(name, time);
+
+#if defined(OS_WIN)
+  static const char kEnvVar[] = "CHROME_PRE_READ_EXPERIMENT";
+
+  // The pre-read experiment is Windows specific.
+  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";
+    AddPreReadHistogramTime(uma_name.c_str(), time);
+  }
+#endif
+}
+
 // BrowserMainParts ------------------------------------------------------------
 
 BrowserMainParts::BrowserMainParts(const MainFunctionParams& parameters)
@@ -2026,8 +2069,8 @@
       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