Update from https://crrev.com/303153

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 273 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(FieldTrial);
 };
 
 //------------------------------------------------------------------------------
 // Class with a list of all active field trials.  A trial is active if it has
 // been registered, which includes evaluating its state based on its probaility.
 // Only one instance of this class exists.
 class BASE_EXPORT FieldTrialList {
  public:
   // Specifies whether field trials should be activated (marked as "used"), when
-  // created using |CreateTrialsFromString()|.
+  // created using |CreateTrialsFromString()|. Has no effect on trials that are
+  // prefixed with |kActivationMarker|, which will always be activated."
   enum FieldTrialActivationMode {
     DONT_ACTIVATE_TRIALS,
     ACTIVATE_TRIALS,
   };
 
   // Define a separator character to use when creating a persistent form of an
   // instance.  This is intended for use as a command line argument, passed to a
   // second process to mimic our state (i.e., provide the same group name).
   static const char kPersistentStringSeparator;  // Currently a slash.
 
+  // Define a marker character to be used as a prefix to a trial name on the
+  // command line which forces its activation.
+  static const char kActivationMarker;  // Currently an asterisk.
+
   // Year that is guaranteed to not be expired when instantiating a field trial
   // via |FactoryGetFieldTrial()|.  Set to two years from the build date.
   static int kNoExpirationYear;
 
   // Observer is notified when a FieldTrial's group is selected.
   class BASE_EXPORT Observer {
    public:
     // Notify observers when FieldTrials's group is selected.
     virtual void OnFieldTrialGroupFinalized(const std::string& trial_name,
                                             const std::string& group_name) = 0;
@@ skipping 90 matching lines @@
   // not been disabled.
   static void GetActiveFieldTrialGroups(
       FieldTrial::ActiveGroups* active_groups);
 
   // Use a state string (re: StatesToString()) to augment the current list of
   // field trials to include the supplied trials, and using a 100% probability
   // for each trial, force them to have the same group string. This is commonly
   // used in a non-browser process, to carry randomly selected state in a
   // browser process into this non-browser process, but could also be invoked
   // through a command line argument to the browser process. The created field
-  // trials are marked as "used" for the purposes of active trial reporting if
-  // |mode| is ACTIVATE_TRIALS. Trial names in |ignored_trial_names| are ignored
-  // when parsing |prior_trials|.
+  // trials are all marked as "used" for the purposes of active trial reporting
+  // if |mode| is ACTIVATE_TRIALS, otherwise each trial will be marked as "used"
+  // if it is prefixed with |kActivationMarker|. Trial names in
+  // |ignored_trial_names| are ignored when parsing |prior_trials|.
   static bool CreateTrialsFromString(
       const std::string& prior_trials,
       FieldTrialActivationMode mode,
       const std::set<std::string>& ignored_trial_names);
 
   // Create a FieldTrial with the given |name| and using 100% probability for
   // the FieldTrial, force FieldTrial to have the same group string as
   // |group_name|. This is commonly used in a non-browser process, to carry
   // randomly selected state in a browser process into this non-browser process.
   // It returns NULL if there is a FieldTrial that is already registered with
@@ skipping 50 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_