Re-enable pre-read experiment as a finch field trial.

chrome/browser/chrome_browser_field_trials_desktop.cc

Index: chrome/browser/chrome_browser_field_trials_desktop.cc
diff --git a/chrome/browser/chrome_browser_field_trials_desktop.cc b/chrome/browser/chrome_browser_field_trials_desktop.cc
index 7096ba830b78c036b6605f8fb30b835dae9dc491..f801165c3bca3c14233a9a6d4584ddca949ca9a8 100644
--- a/chrome/browser/chrome_browser_field_trials_desktop.cc
+++ b/chrome/browser/chrome_browser_field_trials_desktop.cc
@@ -9,6 +9,7 @@
 #include "apps/field_trial_names.h"
 #include "apps/pref_names.h"
 #include "base/command_line.h"
+#include "base/environment.h"
 #include "base/metrics/field_trial.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/string_util.h"
@@ -20,6 +21,7 @@
 #include "chrome/browser/profiles/profiles_state.h"
 #include "chrome/browser/safe_browsing/safe_browsing_blocking_page.h"
 #include "chrome/browser/ui/sync/one_click_signin_helper.h"
+#include "chrome/common/chrome_constants.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/chrome_version_info.h"
 #include "chrome/common/metrics/variations/variations_util.h"
@@ -125,6 +127,44 @@ void SetupLowLatencyFlashAudioFieldTrial() {
     trial->AppendGroup(content::kLowLatencyFlashAudioFieldTrialEnabledName, 25);
 }
 
+void SetupPreReadFieldTrial() {
+  // The chrome executable will have set (or not) an environment variable with
+  // the group name into which this client belongs.
+  std::string group;
+  scoped_ptr<base::Environment> env(base::Environment::Create());
+  env->GetVar(chrome::kPreReadEnvironmentVariable, &group);

[Alexei Svitkine (slow), 2013/09/03 14:59:16]

Can you exit early if this env var isn't set (and not create the trial at all)?

This way, if the other bit of code in client_util.cc doesn't run (for whatever
reason), we won't be putting folks into any group.

Alternatively, make the default group of the experiment "No-Preread" - so that
if the client_util.cc code isn't hit for whatever reason - then that indicates
the preread codepath wasn't hit and thus there was no preread.

[Roger McFarlane (Chromium), 2013/09/03 18:38:43]

The default isn't no-preread, it's full pre-read. the experiment is to find a
hypothesized less-than-full pre-read value that's better than 100%

+
+  // Initialize the field trial. We declare all of the groups here (so that
+  // the dashboard creation tools can find them) but force the probability
+  // of being assigned to the group already chosen by the executable, if any,
+  // to 100%.
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "BrowserPreReadExperiment", 100, "100%-Default",

[Alexei Svitkine (slow), 2013/09/03 14:59:16]

We generally discourage special characters in the field trial names and group
names.

I'm not sure if % specifically will cause any issues, but to be safe, I'd use
something else, like the word "percent" or "pct".

[Roger McFarlane (Chromium), 2013/09/03 18:38:43]

Done.

+          2014, 7, 1, base::FieldTrial::SESSION_RANDOMIZED, NULL));
+  trial->AppendGroup("100%-Control", group == "100%-Control" ? 100 : 0);
+  trial->AppendGroup("95%", group == "95%" ? 100 : 0);
+  trial->AppendGroup("90%", group == "90%" ? 100 : 0);
+  trial->AppendGroup("85%", group == "85%" ? 100 : 0);
+  trial->AppendGroup("80%", group == "80%" ? 100 : 0);
+  trial->AppendGroup("75%", group == "75%" ? 100 : 0);
+  trial->AppendGroup("70%", group == "70%" ? 100 : 0);
+  trial->AppendGroup("65%", group == "65%" ? 100 : 0);
+  trial->AppendGroup("60%", group == "60%" ? 100 : 0);
+  trial->AppendGroup("55%", group == "55%" ? 100 : 0);
+  trial->AppendGroup("50%", group == "50%" ? 100 : 0);
+  trial->AppendGroup("45%", group == "45%" ? 100 : 0);
+  trial->AppendGroup("40%", group == "40%" ? 100 : 0);
+  trial->AppendGroup("35%", group == "35%" ? 100 : 0);
+  trial->AppendGroup("30%", group == "30%" ? 100 : 0);
+  trial->AppendGroup("25%", group == "25%" ? 100 : 0);
+  trial->AppendGroup("20%", group == "20%" ? 100 : 0);
+  trial->AppendGroup("15%", group == "15%" ? 100 : 0);
+  trial->AppendGroup("10%", group == "10%" ? 100 : 0);
+  trial->AppendGroup("5%", group == "5%" ? 100 : 0);
+  trial->AppendGroup("0%", group == "0%" ? 100 : 0);

[Alexei Svitkine (slow), 2013/09/03 14:59:16]

You need to call trial->group() on the experiment for it to be marked as used.
Please do it here.

[Roger McFarlane (Chromium), 2013/09/03 18:38:43]

Done.

+}
+
 }  // namespace
 
 void SetupDesktopFieldTrials(const CommandLine& parsed_command_line,
@@ -139,6 +179,7 @@ void SetupDesktopFieldTrials(const CommandLine& parsed_command_line,
   DisableShowProfileSwitcherTrialIfNecessary();
   SetupAppLauncherFieldTrial(local_state);
   SetupLowLatencyFlashAudioFieldTrial();
+  SetupPreReadFieldTrial();
 }
 
 }  // namespace chrome