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

components/metrics/file_metrics_provider.h

Index: components/metrics/file_metrics_provider.h
diff --git a/components/metrics/file_metrics_provider.h b/components/metrics/file_metrics_provider.h
index 080a8211dd2c0de10bc5c6d06ae6b94a6a462513..001fe837c2e752a857d2cff7d4c4acccbf14ce58 100644
--- a/components/metrics/file_metrics_provider.h
+++ b/components/metrics/file_metrics_provider.h
@@ -90,8 +90,9 @@ class FileMetricsProvider : public MetricsProvider,
   // 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,