Add UMA for service worker navigation preload.

content/test/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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
@@ skipping 1217 matching lines @@
     "../browser/service_worker/link_header_support_unittest.cc",
     "../browser/service_worker/service_worker_cache_writer_unittest.cc",
     "../browser/service_worker/service_worker_context_core_unittest.cc",
     "../browser/service_worker/service_worker_context_request_handler_unittest.cc",
     "../browser/service_worker/service_worker_context_unittest.cc",
     "../browser/service_worker/service_worker_controllee_request_handler_unittest.cc",
     "../browser/service_worker/service_worker_database_unittest.cc",
     "../browser/service_worker/service_worker_dispatcher_host_unittest.cc",
     "../browser/service_worker/service_worker_handle_unittest.cc",
     "../browser/service_worker/service_worker_job_unittest.cc",
+    "../browser/service_worker/service_worker_metrics_unittest.cc",
     "../browser/service_worker/service_worker_process_manager_unittest.cc",
     "../browser/service_worker/service_worker_provider_host_unittest.cc",
     "../browser/service_worker/service_worker_read_from_cache_job_unittest.cc",
     "../browser/service_worker/service_worker_registration_unittest.cc",
     "../browser/service_worker/service_worker_request_handler_unittest.cc",
     "../browser/service_worker/service_worker_storage_unittest.cc",
     "../browser/service_worker/service_worker_url_request_job_unittest.cc",
     "../browser/service_worker/service_worker_version_unittest.cc",
     "../browser/service_worker/service_worker_write_to_cache_job_unittest.cc",
     "../browser/shareable_file_reference_unittest.cc",
@@ skipping 530 matching lines @@
   if (is_android) {
     deps += [ "//testing/android/native_test:native_test_native_code" ]
   }
 }
 
 group("fuzzers") {
   deps = [
     "//content/test/fuzzer",
   ]
 }