Plumb trial association without overriding a feature to subprocesses.

base/feature_list.h

 // Copyright 2015 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 BASE_FEATURE_LIST_H_
 #define BASE_FEATURE_LIST_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 66 matching lines @@
 class BASE_EXPORT FeatureList {
  public:
   FeatureList();
   ~FeatureList();
 
   // Initializes feature overrides via command-line flags |enable_features| and
   // |disable_features|, each of which is a comma-separated list of features to
   // enable or disable, respectively. If a feature appears on both lists, then
   // it will be disabled. If a list entry has the format "FeatureName<TrialName"
   // then this initialization will also associate the feature state override
-  // with the named field trial, if it exists. Must only be invoked during the
-  // initialization phase (before FinalizeInitialization() has been called).
+  // with the named field trial, if it exists. If a feature name is prefixed
+  // with the '*' character, it will be created with OVERRIDE_USE_DEFAULT -
+  // which is useful for associating with a trial while using the default state.
+  // Must only be invoked during the initialization phase (before
+  // FinalizeInitialization() has been called).
   void InitializeFromCommandLine(const std::string& enable_features,
                                  const std::string& disable_features);
 
   // Specifies whether a feature override enables or disables the feature.
   enum OverrideState {
     OVERRIDE_USE_DEFAULT,
     OVERRIDE_DISABLE_FEATURE,
     OVERRIDE_ENABLE_FEATURE,
   };
 
@@ skipping 20 matching lines @@
   // called but before the instance is registered via SetInstance().
   void RegisterFieldTrialOverride(const std::string& feature_name,
                                   OverrideState override_state,
                                   FieldTrial* field_trial);
 
   // Returns comma-separated lists of feature names (in the same format that is
   // accepted by InitializeFromCommandLine()) corresponding to features that
   // have been overridden - either through command-line or via FieldTrials. For
   // those features that have an associated FieldTrial, the output entry will be
   // of the format "FeatureName<TrialName", where "TrialName" is the name of the
-  // FieldTrial. Must be called only after the instance has been initialized and
-  // registered.
+  // FieldTrial. Features that have overrides with OVERRIDE_USE_DEFAULT will be
+  // added to |enable_overrides| with a '*' character prefix. Must be called
+  // only after the instance has been initialized and registered.
   void GetFeatureOverrides(std::string* enable_overrides,
                            std::string* disable_overrides);
 
   // Returns whether the given |feature| is enabled. Must only be called after
   // the singleton instance has been registered via SetInstance(). Additionally,
   // a feature with a given name must only have a single corresponding Feature
   // struct, which is checked in builds with DCHECKs enabled.
   static bool IsEnabled(const Feature& feature);
 
   // Splits a comma-separated string containing feature names into a vector.
@@ skipping 89 matching lines @@
   // Whether this object has been fully initialized. This gets set to true as a
   // result of FinalizeInitialization().
   bool initialized_;
 
   DISALLOW_COPY_AND_ASSIGN(FeatureList);
 };
 
 }  // namespace base
 
 #endif  // BASE_FEATURE_LIST_H_