headless: Remove old load observer API

headless/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.
 
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 group("headless") {
   deps = [
@@ skipping 311 matching lines @@
     "//base",
     "//content/test:browsertest_base",
     "//content/test:test_support",
     "//headless:headless_lib",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 executable("headless_shell") {
-  testonly = true
-
   sources = [
     "app/headless_shell.cc",
     "app/headless_shell_switches.cc",
     "app/headless_shell_switches.h",
   ]
 
   deps = [
     "//headless:headless_lib",
   ]
 }