Convert startup_metric_utils to use mojo.

components/startup_metric_utils/browser/BUILD.gn

 # Copyright 2013 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.
 
 # We have 2 separate targets because //components/html_viewer requires :lib,
 # but has symbols that conflict with mojo symbols that :message_filter_lib
 # indirectly depends on.
 
 group("browser") {
   testonly = true
 
   deps = [
+    ":host",
     ":lib",
-    ":message_filter_lib",
   ]
 }
 
 source_set("lib") {
   sources = [
     "pref_names.cc",
     "pref_names.h",
     "startup_metric_utils.cc",
     "startup_metric_utils.h",
   ]
 
   deps = [
     "//base",
     "//components/prefs",
     "//components/version_info",
   ]
 }
 
-source_set("message_filter_lib") {
+source_set("host") {
   sources = [
-    "startup_metric_message_filter.cc",
-    "startup_metric_message_filter.h",
+    "startup_metric_host_impl.cc",
+    "startup_metric_host_impl.h",
   ]
 
   deps = [
     ":lib",
     "//base",
-    "//components/startup_metric_utils/common",
+    "//components/startup_metric_utils/common:interfaces",
     "//content/public/browser",
   ]
 }