Make --force-fieldtrials not activate them in the browser process.

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 374 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.
-  static bool CreateTrialsFromString(const std::string& prior_trials);
+  // trials are marked as "used" for the purposes of active trial reporting if
+  // |activate_trials| is true.
+  static bool CreateTrialsFromString(const std::string& prior_trials,
+                                     bool activate_trials);

[Ilya Sherman, 2013/06/28 00:58:07]

nit: Please define an enumerated constant for this, rather than passing in a
boolean, which is hard to interpret at the call sites.  Alternately, since there
seems to be only one client of this code that actually wants to activate all of
the field trials, maybe it makes sense to move the activation logic out of this
method?

[Alexei Svitkine (slow), 2013/06/28 17:05:15]

Done.

 
   // 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
   // the same |name| but has different finalized group string (|group_name|).
   static FieldTrial* CreateFieldTrial(const std::string& name,
                                       const std::string& group_name);
 
@@ skipping 46 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_