Add three missing test files to gn builds

components/metrics/BUILD.gn

 # 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.
 
 declare_args() {
   # Overrides os name in uma metrics log to "Blimp".
   metrics_use_blimp = false
 }
 
 # GYP version: components/metrics.gypi:metrics
@@ skipping 302 matching lines @@
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "call_stack_profile_metrics_provider_unittest.cc",
     "cloned_install_detector_unittest.cc",
     "daily_event_unittest.cc",
     "data_use_tracker_unittest.cc",
     "drive_metrics_provider_unittest.cc",
+    "file_metrics_provider_unittest.cc",
     "histogram_encoder_unittest.cc",
     "machine_id_provider_win_unittest.cc",
     "metrics_log_manager_unittest.cc",
     "metrics_log_unittest.cc",
     "metrics_reporting_scheduler_unittest.cc",
     "metrics_service_unittest.cc",
     "metrics_state_manager_unittest.cc",
     "net/net_metrics_log_uploader_unittest.cc",
     "persisted_logs_unittest.cc",
     "profiler/profiler_metrics_provider_unittest.cc",
@@ skipping 29 matching lines @@
   # These are only used by the blimp team, which has entirely migrated to gn,
   # so this logic is not replicated in the gyp file.
   if (metrics_use_blimp) {
     defines = [
       "OVERRIDE_OS_NAME_TO_BLIMP",
       "ENABLE_REPORTING_BLIMP",
     ]
   }
 }
 # TODO(GYP): metrics_chromeos