Add variations params api to get params from features.

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 <memory>
 #include <string>
@@ skipping 123 matching lines @@
   // 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);
 
+  // Returns the field trial associated with the given |feature|. Must only be
+  // called after the singleton instance has been registered via SetInstance().
+  static FieldTrial* GetFieldTrial(const Feature& feature);

[Alexei Svitkine (slow), 2016/05/03 17:04:25]

Can you also add a unit test for this API?

[jwd, 2016/05/03 20:12:27]

Done.

+
   // Splits a comma-separated string containing feature names into a vector.
   static std::vector<std::string> SplitFeatureListString(
       const std::string& input);
 
   // Initializes and sets an instance of FeatureList with feature overrides via
   // command-line flags |enable_features| and |disable_features| if one has not
   // already been set from command-line flags. Returns true if an instance did
   // not previously exist. See InitializeFromCommandLine() for more details
   // about |enable_features| and |disable_features| parameters.
   static bool InitializeInstance(const std::string& enable_features,
@@ skipping 39 matching lines @@
   // Finalizes the initialization state of the FeatureList, so that no further
   // overrides can be registered. This is called by SetInstance() on the
   // singleton feature list that is being registered.
   void FinalizeInitialization();
 
   // Returns whether the given |feature| is enabled. This is invoked by the
   // public FeatureList::IsEnabled() static function on the global singleton.
   // Requires the FeatureList to have already been fully initialized.
   bool IsFeatureEnabled(const Feature& feature);
 
+  // Returns the field trial associated with the given |feature|. This is
+  // invoked by the public FeatureList::GetFieldTrial() static function on the
+  // global singleton. Requires the FeatureList to have already been fully
+  // initialized.
+  base::FieldTrial* GetAssociatedFieldTrial(const Feature& feature);
+
   // For each feature name in comma-separated list of strings |feature_list|,
   // registers an override with the specified |overridden_state|. Also, will
   // associate an optional named field trial if the entry is of the format
   // "FeatureName<TrialName".
   void RegisterOverridesFromCommandLine(const std::string& feature_list,
                                         OverrideState overridden_state);
 
   // Registers an override for feature |feature_name|. The override specifies
   // whether the feature should be on or off (via |overridden_state|), which
   // will take precedence over the feature's default state. If |field_trial| is
@@ skipping 28 matching lines @@
 
   // Whether this object has been initialized from command line.
   bool initialized_from_command_line_;
 
   DISALLOW_COPY_AND_ASSIGN(FeatureList);
 };
 
 }  // namespace base
 
 #endif  // BASE_FEATURE_LIST_H_