Introduce a field trial for Instant.

chrome/browser/instant/instant_field_trial.h

Index: chrome/browser/instant/instant_field_trial.h
diff --git a/chrome/browser/instant/instant_field_trial.h b/chrome/browser/instant/instant_field_trial.h
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+++ b/chrome/browser/instant/instant_field_trial.h
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+// 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.
+
+#ifndef CHROME_BROWSER_INSTANT_INSTANT_FIELD_TRIAL_H_
+#define CHROME_BROWSER_INSTANT_INSTANT_FIELD_TRIAL_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+
+class Profile;
+
+// This class manages the Instant field trial. Each user is in exactly one of
+// three field trial groups: Inactive, Control or Experiment.
+// - Inactive users are those who have played with the Instant option in the
+//   Preferences page, or those for whom group policy provides an override, or
+//   those with an incognito profile, etc. The field trial is inactive for such
+//   users, so their Instant preference setting is respected.
+// - Control and Experiment are all the other users, i.e., those who have never
+//   touched the Preferences option. Some percentage of these users are chosen
+//   into the Experiment group and get Instant enabled automatically. The rest
+//   fall into the Control group; for them, Instant remains disabled by default.
+// - Control and Experiment are further split into two subgroups each, in order
+//   to detect bias between them (when analyzing metrics). The subgroups are
+//   treated identically for all other purposes.
+class InstantFieldTrial {
+ public:
+  enum Group {
+    INACTIVE,
+    CONTROL1,
+    CONTROL2,
+    EXPERIMENT1,
+    EXPERIMENT2,
+  };
+  static Group GetGroup(Profile* profile);
+
+  // Convenience method.
+  static bool IsExperimentGroup(Profile* profile) {
+    Group group = GetGroup(profile);
+    return group == EXPERIMENT1 || group == EXPERIMENT2;
+  }
+
+  // Returns a string describing the user's group. Can be added to histogram
+  // names, to split histograms by field trial groups.
+  static std::string GetGroupName(Profile* profile);
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(InstantFieldTrial);
+};
+
+#endif  // CHROME_BROWSER_INSTANT_INSTANT_FIELD_TRIAL_H_