Add an override flag to enable metrics reporting.

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 552 matching lines @@
 bool ChromeMetricsServiceClient::IsReportingPolicyManaged() {
   return IsMetricsReportingPolicyManaged();
 }
 
 metrics::EnableMetricsDefault
 ChromeMetricsServiceClient::GetMetricsReportingDefaultState() {
   return metrics::GetMetricsReportingDefaultState(
       g_browser_process->local_state());
 }
 
+// static
+bool ChromeMetricsServiceClient::IsMetricsReportingEnabledForTesting() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kEnableMetricsReportingForTesting);
+}
+
 void ChromeMetricsServiceClient::Initialize() {
   PrefService* local_state = g_browser_process->local_state();
 
   // Clear deprecated metrics preference for Android.
   // TODO(gayane): Cleanup this code after M60 when the pref would be cleared
   // from clients.
 #if defined(OS_ANDROID)
   local_state->ClearPref(prefs::kCrashReportingEnabled);
 #endif
 
   metrics_service_.reset(
       new metrics::MetricsService(metrics_state_manager_, this, local_state));
 
   RegisterMetricsServiceProviders();
 
-  if (base::FeatureList::IsEnabled(ukm::kUkmFeature)) {
+  if (IsMetricsReportingEnabledForTesting() ||
+      base::FeatureList::IsEnabled(ukm::kUkmFeature)) {
     ukm_service_.reset(new ukm::UkmService(local_state, this));
     RegisterUKMProviders();
   }
 }
 
 void ChromeMetricsServiceClient::RegisterMetricsServiceProviders() {
   PrefService* local_state = g_browser_process->local_state();
 
   // Gets access to persistent metrics shared by sub-processes.
   metrics_service_->RegisterMetricsProvider(
@@ skipping 382 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();
 }