No 'prefs' state is necessary for an active file.

components/metrics/file_metrics_provider.h

 // Copyright 2016 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_METRICS_FILE_METRICS_PROVIDER_H_
 #define COMPONENTS_METRICS_FILE_METRICS_PROVIDER_H_
 
 #include <list>
 #include <memory>
 #include <string>
@@ skipping 72 matching lines @@
   };
 
   FileMetricsProvider(const scoped_refptr<base::TaskRunner>& task_runner,
                       PrefService* local_state);
   ~FileMetricsProvider() override;
 
   // Indicates a file or directory to be monitored and how the file or files
   // within that directory are used. Because some metadata may need to persist
   // across process restarts, preferences entries are used based on the
   // |prefs_key| name. Call RegisterPrefs() with the same name to create the
-  // necessary keys in advance. Set |prefs_key| empty if no persistence is
-  // required.
+  // necessary keys in advance. Set |prefs_key| empty (nullptr will work) if
+  // no persistence is required. ACTIVE files shouldn't have a pref key as
+  // they update internal state about what has been previously sent.
   void RegisterSource(const base::FilePath& path,
                       SourceType type,
                       SourceAssociation source_association,
                       const base::StringPiece prefs_key);
 
   // Registers all necessary preferences for maintaining persistent state
   // about a monitored file across process restarts. The |prefs_key| is
   // typically the filename.
   static void RegisterPrefs(PrefRegistrySimple* prefs,
                             const base::StringPiece prefs_key);
@@ skipping 94 matching lines @@
 
   base::ThreadChecker thread_checker_;
   base::WeakPtrFactory<FileMetricsProvider> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FileMetricsProvider);
 };
 
 }  // namespace metrics
 
 #endif  // COMPONENTS_METRICS_FILE_METRICS_PROVIDER_H_