Add an override flag to enable metrics reporting.

components/metrics_services_manager/BUILD.gn

 # Copyright 2015 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.
 
 static_library("metrics_services_manager") {
   sources = [
     "metrics_services_manager.cc",
     "metrics_services_manager.h",
+    "metrics_services_manager_client.cc",
     "metrics_services_manager_client.h",
   ]
 
   deps = [
     "//base",
     "//components/metrics",
     "//components/rappor",
     "//components/ukm",
     "//components/variations",
     "//components/variations/service",
   ]
 }