Introduce a field trial for Instant.

chrome/browser/instant/instant_field_trial.h

+// 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>
+
+class Profile;
+
+namespace instant_field_trial {
+
+// Sets up the Instant field trial (creates groups, registers the field trial
+// with the global FieldTrialList and selects the user into one of the groups).
+// MUST NOT BE CALLED MORE THAN ONCE.
+void Init();
+
+// Returns true if the user is part of the field trial experiment group.

[sky, 2011/07/12 00:37:50]

Why do we need both IsExperiment and IsControl. Isn't it the case that if you
are either in the experiment or not in the experiment group?

I haven't looked at experiment code before, so I could totally be wrong. Either
way, could you clarify what it means to be in the experiment group and control
group.

[sreeram, 2011/07/12 03:36:18]

We need to be able to distinguish among these states:
1. Field trial is active, the user is in the experiment group (i.e., instant
enabled).
2. Field trial is active, the user is in the control group (i.e., instant
disabled).
3. Field trial is inactive (because the user has explicitly enabled/disabled the
preference, or because of group policy override, or because of incognito mode,
etc).

We want to log metrics for groups 1 and 2, but not for 3 (i.e., we want to avoid
those self-selected users). That's why we need both IsExperiment() (group 1) and
IsControl() (group 2).

+bool IsExperiment(Profile* profile);
+
+// Returns true if the user is part of the field trial control group.
+bool IsControl(Profile* profile);
+
+// Returns a string that identifies the group the user has been selected into.
+// Useful to split histograms by field trial groups.
+std::string GetGroupName(Profile* profile);
+
+}  // namespace instant_field_trial
+
+#endif  // CHROME_BROWSER_INSTANT_INSTANT_FIELD_TRIAL_H_