Notification: Implement two-leveled notification.

ui/message_center/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("//testing/test.gni")
 import("//ui/vector_icons/vector_icons.gni")
 
 aggregate_vector_icons("message_center_vector_icons") {
   icon_directory = "vector_icons"
 
   icons = [
+    "notification_close_button.1x.icon",
+    "notification_close_button.icon",
+    "notification_expand_less.icon",
+    "notification_expand_more.icon",
     "notification_settings_button.1x.icon",
     "notification_settings_button.icon",
-    "notification_close_button.1x.icon",
-    "notification_close_button.icon",
     "product.1x.icon",
     "product.icon",
   ]
 }
 
 # TODO(msw|mukai|dewittj): Move ash-specific files: crbug.com/585175
 component("message_center") {
   deps = [
     "//base",
     "//ui/base",
@@ skipping 106 matching lines @@
         "views/message_popup_collection.cc",
         "views/message_popup_collection.h",
         "views/message_view.cc",
         "views/message_view.h",
         "views/message_view_context_menu_controller.cc",
         "views/message_view_context_menu_controller.h",
         "views/message_view_factory.cc",
         "views/message_view_factory.h",
         "views/notification_button.cc",
         "views/notification_button.h",
+        "views/notification_header_view.cc",
+        "views/notification_header_view.h",
         "views/notification_view.cc",
         "views/notification_view.h",
         "views/notification_view_md.cc",
         "views/notification_view_md.h",
         "views/notifier_settings_view.cc",
         "views/notifier_settings_view.h",
         "views/padded_button.cc",
         "views/padded_button.h",
         "views/popup_alignment_delegate.cc",
         "views/popup_alignment_delegate.h",
@@ skipping 124 matching lines @@
     if (!is_ios) {
       sources += [ "mojo/struct_traits_unittest.cc" ]
       deps += [
         "//mojo/edk/system",
         "//ui/gfx:test_support",
         "//ui/message_center/mojo:test_interfaces",
       ]
     }
   }  # !is_ios && !is_android
 }