Add an override flag to enable metrics reporting.

chrome/browser/metrics/chrome_metrics_service_client.h

Index: chrome/browser/metrics/chrome_metrics_service_client.h
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.h b/chrome/browser/metrics/chrome_metrics_service_client.h
index a482362329c762314db93268cd27b213c937ac97..e973b8342dd9d39db57888dbc1eb226c7c10d468 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.h
+++ b/chrome/browser/metrics/chrome_metrics_service_client.h
@@ -64,6 +64,9 @@ class ChromeMetricsServiceClient : public metrics::MetricsServiceClient,
   // Registers local state prefs used by this class.
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
+  // Checks if the user has forced metrics collection on via the override flag.
+  static bool IsMetricsReportingEnabledForTesting();
+
   // metrics::MetricsServiceClient:
   metrics::MetricsService* GetMetricsService() override;
   ukm::UkmService* GetUkmService() override;