Add an override flag to enable metrics reporting.

chrome/browser/metrics/chrome_metrics_service_client.cc

Index: chrome/browser/metrics/chrome_metrics_service_client.cc
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.cc b/chrome/browser/metrics/chrome_metrics_service_client.cc
index 66bb833bf80dad3eee8256265a21252195cb192f..4bcd0c3c331b11aec915b26c66ad3deece6bed7f 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.cc
+++ b/chrome/browser/metrics/chrome_metrics_service_client.cc
@@ -570,6 +570,12 @@ ChromeMetricsServiceClient::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();
 
@@ -585,7 +591,8 @@ void ChromeMetricsServiceClient::Initialize() {
 
   RegisterMetricsServiceProviders();
 
-  if (base::FeatureList::IsEnabled(ukm::kUkmFeature)) {
+  if (IsMetricsReportingEnabledForTesting() ||
+      base::FeatureList::IsEnabled(ukm::kUkmFeature)) {
     ukm_service_.reset(new ukm::UkmService(local_state, this));
     RegisterUKMProviders();
   }