Add unit tests target to GN build.

extensions/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("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 assert(enable_extensions)
 
@@ skipping 57 matching lines @@
   testonly = true
   sources = [
     "browser/api/dns/mock_host_resolver_creator.cc",
     "browser/api/dns/mock_host_resolver_creator.h",
     "browser/api/storage/settings_test_util.cc",
     "browser/api/storage/settings_test_util.h",
     "browser/api_test_utils.cc",
     "browser/api_test_utils.h",
     "browser/api_unittest.cc",
     "browser/api_unittest.h",
+    "browser/app_window/test_app_window_contents.cc",
+    "browser/app_window/test_app_window_contents.h",
+    "browser/extension_error_test_util.cc",
+    "browser/extension_error_test_util.h",
     "browser/extensions_test.cc",
     "browser/extensions_test.h",
     "browser/guest_view/test_guest_view_manager.cc",
     "browser/guest_view/test_guest_view_manager.h",
     "browser/mock_extension_system.cc",
     "browser/mock_extension_system.h",
     "browser/test_extension_registry_observer.cc",
     "browser/test_extension_registry_observer.h",
     "browser/test_extensions_browser_client.cc",
     "browser/test_extensions_browser_client.h",
@@ skipping 262 matching lines @@
   ]
 
   if (is_win) {
     deps += [ "//base/allocator" ]
   }
 
   if (is_mac) {
     deps += [ "shell:app_shell" ]  # Needed for App Shell.app's Helper.
   }
 }