Disable sending persistent histograms from last session by default.

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 192 matching lines @@
           ->GetTaskRunnerWithShutdownBehavior(
               base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN);
 
   // Create an object to monitor files of metrics and include them in reports.
   std::unique_ptr<metrics::FileMetricsProvider> file_metrics_provider(
       new metrics::FileMetricsProvider(task_runner,
                                        g_browser_process->local_state()));
 
   base::FilePath user_data_dir;
   if (base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir)) {
-    // Register the file holding browser metrics.
+    // Reporting of persistent histograms from last session is controlled by
+    // a feature param. TODO(bcwhite): The current default is not to upload
+    // until some issues are resolved. See crbug.com/706422 for details.
+    std::string send_unreported = variations::GetVariationParamValueByFeature(
+        base::kPersistentHistogramsFeature, "send_unreported_metrics");
+    boolean report_previous_persistent_histograms =

[bcwhite, 2017/03/29 16:09:12]

boolean => bool

[Alexei Svitkine (slow), 2017/03/29 16:19:01]

Oops, teaches for sending right before lunch before having a chance to compile.
Done.

+        metrics_reporting_enabled && (send_unreported == "yes");
     RegisterOrRemovePreviousRunMetricsFile(
-        metrics_reporting_enabled, user_data_dir,
+        report_previous_persistent_histograms, user_data_dir,
         ChromeMetricsServiceClient::kBrowserMetricsName, task_runner,
         file_metrics_provider.get());
 
     // Register the Crashpad metrics files.
     // Register the data from the previous run if crashpad_handler didn't exit
     // cleanly.
     RegisterOrRemovePreviousRunMetricsFile(
         metrics_reporting_enabled, user_data_dir,
         kCrashpadHistogramAllocatorName, task_runner,
         file_metrics_provider.get());
@@ skipping 741 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();
 }