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

components/metrics/file_metrics_provider.cc

 // 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.
 
 #include "components/metrics/file_metrics_provider.h"
 
 #include "base/command_line.h"
 #include "base/files/file.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
@@ skipping 123 matching lines @@
                                          const base::StringPiece prefs_key) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
   // Ensure that kSourceOptions has been filled for this type.
   DCHECK_GT(arraysize(kSourceOptions), static_cast<size_t>(type));
 
   std::unique_ptr<SourceInfo> source(new SourceInfo(type));
   source->prefs_key = prefs_key.as_string();
 
   switch (source->type) {
+    case SOURCE_HISTOGRAMS_ACTIVE_FILE:
+      DCHECK(prefs_key.empty());
+    // fall through
     case SOURCE_HISTOGRAMS_ATOMIC_FILE:
-    case SOURCE_HISTOGRAMS_ACTIVE_FILE:
       source->path = path;
       break;
     case SOURCE_HISTOGRAMS_ATOMIC_DIR:
       source->directory = path;
       break;
   }
 
   // |prefs_key| may be empty if the caller does not wish to persist the
   // state across instances of the program.
   if (pref_service_ && !prefs_key.empty()) {
@@ skipping 404 matching lines @@
   // important to know how much total "jank" may be introduced.
   SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.SnapshotTime.Total");
 
   for (std::unique_ptr<SourceInfo>& source : sources_mapped_) {
     SCOPED_UMA_HISTOGRAM_TIMER("UMA.FileMetricsProvider.SnapshotTime.File");
     MergeHistogramDeltasFromSource(source.get());
   }
 }
 
 }  // namespace metrics