Add UMA metric: Startup.BrowserMainToRendererMain.

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 :browser,
+# but has symbols that conflict with mojo symbols that :message_filter
+# indirectly depends on.
+
 source_set("browser") {
   sources = [
     "startup_metric_utils.cc",
     "startup_metric_utils.h",
   ]
 
   deps = [
     "//base",
   ]
 }
+
+source_set("message_filter") {
+  sources = [
+    "startup_metric_message_filter.cc",
+    "startup_metric_message_filter.h",
+  ]
+
+  deps = [
+    ":browser",
+    "//base",
+    "//components/startup_metric_utils/common",
+    "//content/public/browser",
+  ]
+}