Pull MessageListView into its own file for easier testing.

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")
 
 component("message_center") {
   deps = [
@@ skipping 76 matching lines @@
         "views/bounded_label.cc",
         "views/bounded_label.h",
         "views/constants.h",
         "views/desktop_popup_alignment_delegate.cc",
         "views/desktop_popup_alignment_delegate.h",
         "views/message_center_button_bar.cc",
         "views/message_center_button_bar.h",
         "views/message_center_controller.h",
         "views/message_center_view.cc",
         "views/message_center_view.h",
+        "views/message_list_view.cc",
+        "views/message_list_view.h",
         "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/notification_button.cc",
         "views/notification_button.h",
         "views/notification_view.cc",
         "views/notification_view.h",
@@ skipping 122 matching lines @@
       deps += [
         # Compositor is needed by message_center_view_unittest.cc and for the
         # fonts used by bounded_label_unittest.cc.
         "//ui/compositor",
         "//ui/views",
         "//ui/views:test_support",
       ]
     }
   }  # enable_notifications && !is_android
 }