Introduce a field trial for Instant.

chrome/browser/instant/instant_field_trial.cc

+// 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.
+
+#include "chrome/browser/instant/instant_field_trial.h"
+
+#include "chrome/browser/prefs/pref_service.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/common/pref_names.h"
+
+// static
+InstantFieldTrial::Group InstantFieldTrial::GetGroup(Profile* profile) {
+  if (profile->IsOffTheRecord())
+    return INACTIVE;
+
+  const PrefService* prefs = profile->GetPrefs();
+  if (!prefs ||
+      prefs->GetBoolean(prefs::kInstantEnabledOnce) ||
+      prefs->IsManagedPreference(prefs::kInstantEnabled)) {
+    return INACTIVE;
+  }
+
+  const double random = prefs->GetDouble(prefs::kInstantFieldTrialRandomDraw);

[jar (doing other things), 2011/07/22 01:42:17]

IMO, you should use persistent prefs, and (as you noted), you'll only get to run
the tests on folks that have opt'ed into UMA.  Writing a random number into the
prefs is indeed marking the build with a unique ID, and as noted, that is
undesirable from a privacy perspective for some users.

[sreeram, 2011/07/22 14:31:16]

Addressed by quantizing the random number.

+  return random < 0.45 ? CONTROL1 :    // 45%
+         random < 0.90 ? CONTROL2 :    // 45%
+         random < 0.95 ? EXPERIMENT1   //  5%
+                       : EXPERIMENT2;  //  5%
+}
+
+// static
+std::string InstantFieldTrial::GetGroupName(Profile* profile) {
+  switch (GetGroup(profile)) {
+    case INACTIVE:    return "InstantInactive";
+    case CONTROL1:    return "InstantControl1";
+    case CONTROL2:    return "InstantControl2";
+    case EXPERIMENT1: return "InstantExperiment1";
+    case EXPERIMENT2: return "InstantExperiment2";
+  }
+  NOTREACHED();
+  return "InstantUnknown";
+}