Notification: Implement two-leveled notification.

ui/message_center/BUILD.gn

Index: ui/message_center/BUILD.gn
diff --git a/ui/message_center/BUILD.gn b/ui/message_center/BUILD.gn
index 52b0631224d28e90c70a99e60bb3021e42929fd0..161038a933e122f7f35608a9b9de39388d739fba 100644
--- a/ui/message_center/BUILD.gn
+++ b/ui/message_center/BUILD.gn
@@ -11,10 +11,12 @@ aggregate_vector_icons("message_center_vector_icons") {
   icon_directory = "vector_icons"
 
   icons = [
-    "notification_settings_button.1x.icon",
-    "notification_settings_button.icon",
     "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",
     "product.1x.icon",
     "product.icon",
   ]
@@ -141,6 +143,8 @@ component("message_center") {
         "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",