content: Port layout_test_helper target to GN build.

content/shell/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/features.gni")
 import("//build/config/ui.gni")
 import("//tools/grit/repack.gni")
 import("//tools/grit/grit_rule.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
@@ skipping 523 matching lines @@
     }
   }
 } else {
   group("content_shell") {
     testonly = true
     deps = [
       ":content_shell_lib",
     ]
   }
 }
+
+if (is_mac || is_win) {
+  # GYP version: content/content_shell_and_tests.gyp:layout_test_helper
+  executable("layout_test_helper") {
+    sources = [
+      "renderer/test_runner/helper/layout_test_helper_mac.mm",
+      "renderer/test_runner/helper/layout_test_helper_win.cc",
+    ]
+
+    if (is_mac) {
+      libs = [ "AppKit.framework" ]
+    }
+  }
+}