Metrics for feature engagement tracker.

components/feature_engagement_tracker/internal/single_invalid_configuration.h

 // Copyright 2017 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 COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_SINGLE_INVALID_CONFIGURATION_H_
 #define COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_SINGLE_INVALID_CONFIGURATION_H_
 
 #include <unordered_set>
 
 #include "base/macros.h"
 #include "components/feature_engagement_tracker/internal/configuration.h"
 
 namespace base {
 struct Feature;
 }  // namespace base
 
 namespace feature_engagement_tracker {
 
 // An Configuration that always returns the same single invalid configuration,
-// regardless of which feature.
+// regardless of which feature. Also holds an empty ConfigMap.
 class SingleInvalidConfiguration : public Configuration {
  public:
   SingleInvalidConfiguration();
   ~SingleInvalidConfiguration() override;
 
   // Configuration implementation.
   const FeatureConfig& GetFeatureConfig(
       const base::Feature& feature) const override;
+  const Configuration::ConfigMap& GetRegisteredFeatures() const override;
 
  private:
   // The invalid configuration to always return.
   FeatureConfig invalid_feature_config_;
 
+  // An empty map.
+  ConfigMap configs_;
+
   DISALLOW_COPY_AND_ASSIGN(SingleInvalidConfiguration);
 };
 
 }  // namespace feature_engagement_tracker
 
 #endif  // COMPONENTS_FEATURE_ENGAGEMENT_TRACKER_INTERNAL_SINGLE_INVALID_CONFIGURATION_H_