Add browser_tests to the 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("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 assert(enable_extensions)
 
 # GYP version: extensions/extensions_resources.gyp:extensions_resources
@@ skipping 56 matching lines @@
     "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/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",
     "browser/test_management_policy.cc",
     "browser/test_management_policy.h",
     "browser/test_runtime_api_delegate.cc",
     "browser/test_runtime_api_delegate.h",
     "common/extension_builder.cc",
     "common/extension_builder.h",
     "common/manifest_test.cc",
     "common/manifest_test.h",
@@ skipping 182 matching lines @@
       "//testing/gmock",
       "//testing/gtest",
       "//third_party/leveldatabase",
     ]
 
     if (is_win) {
       deps += [ "//base/allocator" ]
     }
   }
 }