Added a SetForced method to allow forcing a new set of group bucketting.

base/metrics/field_trial.h

 // Copyright (c) 2012 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.
 
 // FieldTrial is a class for handling details of statistical experiments
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
 // The simplest example is an experiment to see whether one of two options
 // produces "better" results across our user population.  In that scenario, UMA
@@ skipping 134 matching lines @@
   bool GetSelectedGroup(SelectedGroup* selected_group);
 
   // Helper function for the most common use: as an argument to specify the
   // name of a HISTOGRAM.  Use the original histogram name as the name_prefix.
   static std::string MakeName(const std::string& name_prefix,
                               const std::string& trial_name);
 
   // Enable benchmarking sets field trials to a common setting.
   static void EnableBenchmarking();
 
+  // Set the field trial as forced, meaning that it was setup earlier than
+  // the hard coded registration of the field trial to override it.
+  // This allows the code that was hard coded to register the field trial to
+  // still succeed even though the field trial has already been registered.
+  // This must be called after appending all the groups, since we will make
+  // the group choice here. Note that this is a NOOP for already forced trials.
+  // And, as the rest of the FieldTrial code, this is not thread safe and must
+  // be done from the UI thread.
+  void SetForced();
+
  private:
   // Allow tests to access our innards for testing purposes.
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, Registration);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, AbsoluteProbabilities);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, RemainingProbability);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FiftyFiftyProbability);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, MiddleProbabilities);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, OneWinner);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DisableProbability);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, Save);
@@ skipping 259 matching lines @@
 
   // List of observers to be notified when a group is selected for a FieldTrial.
   scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_FIELD_TRIAL_H_