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

chrome/browser/metrics/chrome_metrics_service_client.cc

 // Copyright 2014 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 "chrome/browser/metrics/chrome_metrics_service_client.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <utility>
@@ skipping 209 matching lines @@
     // cleanly.
     RegisterOrRemovePreviousRunMetricsFile(
         metrics_reporting_enabled, user_data_dir,
         kCrashpadHistogramAllocatorName, task_runner,
         file_metrics_provider.get());
     if (metrics_reporting_enabled) {
       base::FilePath active_path;
       base::GlobalHistogramAllocator::ConstructFilePaths(
           user_data_dir, kCrashpadHistogramAllocatorName, nullptr,
           &active_path);
-      // Register data that will be populated for the current run.
+      // Register data that will be populated for the current run. "Active"
+      // files need an empty "prefs_key" because they update the file itself.
       file_metrics_provider->RegisterSource(
           active_path,
           metrics::FileMetricsProvider::SOURCE_HISTOGRAMS_ACTIVE_FILE,
           metrics::FileMetricsProvider::ASSOCIATE_CURRENT_RUN,
-          kCrashpadHistogramAllocatorName);
+          base::StringPiece());
     }
   }
 
 #if defined(OS_WIN)
   // Read metrics file from setup.exe.
   base::FilePath program_dir;
   base::PathService::Get(base::DIR_EXE, &program_dir);
   file_metrics_provider->RegisterSource(
       program_dir.AppendASCII(installer::kSetupHistogramAllocatorName),
       metrics::FileMetricsProvider::SOURCE_HISTOGRAMS_ATOMIC_DIR,
@@ skipping 714 matching lines @@
     ukm_service_->Purge();
     ukm_service_->ResetClientId();
   }
   // Signal service manager to enable/disable UKM based on new state.
   UpdateRunningServices();
 }
 
 bool ChromeMetricsServiceClient::IsHistorySyncEnabledOnAllProfiles() {
   return SyncDisableObserver::IsHistorySyncEnabledOnAllProfiles();
 }