Split ExtensionTestObserver and move to //extensions.

extensions/BUILD.gn

 # 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)
 
 group("extensions_resources") {
@@ skipping 89 matching lines @@
     "common/permissions/permission_message_test_util.cc",
     "common/permissions/permission_message_test_util.h",
     "common/test_util.cc",
     "common/test_util.h",
     "renderer/test_extensions_renderer_client.cc",
     "renderer/test_extensions_renderer_client.h",
     "test/background_page_watcher.cc",
     "test/background_page_watcher.h",
     "test/extension_test_message_listener.cc",
     "test/extension_test_message_listener.h",
+    "test/extension_test_notification_observer.cc",
+    "test/extension_test_notification_observer.h",
     "test/result_catcher.cc",
     "test/result_catcher.h",
     "test/test_content_utility_client.cc",
     "test/test_content_utility_client.h",
     "test/test_extensions_client.cc",
     "test/test_extensions_client.h",
     "test/test_permission_message_provider.cc",
     "test/test_permission_message_provider.h",
   ]
 
@@ skipping 229 matching lines @@
     "//testing/gtest",
     "//third_party/hunspell",
     "//third_party/icu",
     "//third_party/libpng",
     "//third_party/zlib",
     "//ui/base:test_support",
     "//ui/resources:ui_test_pak",
     "//ui/web_dialogs:test_support",
   ]
 }