Introduce SystemMenuButton for Ash material design

ash/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/base/ui_features.gni")
 
 assert(use_aura)
@@ skipping 328 matching lines @@
     "common/system/tray/fixed_sized_scroll_view.cc",
     "common/system/tray/fixed_sized_scroll_view.h",
     "common/system/tray/hover_highlight_view.cc",
     "common/system/tray/hover_highlight_view.h",
     "common/system/tray/ime_info.cc",
     "common/system/tray/ime_info.h",
     "common/system/tray/label_tray_view.cc",
     "common/system/tray/label_tray_view.h",
     "common/system/tray/special_popup_row.cc",
     "common/system/tray/special_popup_row.h",
+    "common/system/tray/system_menu_button.cc",
+    "common/system/tray/system_menu_button.h",
     "common/system/tray/system_tray.cc",
     "common/system/tray/system_tray.h",
     "common/system/tray/system_tray_bubble.cc",
     "common/system/tray/system_tray_bubble.h",
     "common/system/tray/system_tray_delegate.cc",
     "common/system/tray/system_tray_delegate.h",
     "common/system/tray/system_tray_item.cc",
     "common/system/tray/system_tray_item.h",
     "common/system/tray/system_tray_notifier.cc",
     "common/system/tray/system_tray_notifier.h",
@@ skipping 1286 matching lines @@
   if (is_win) {
     configs -= [ "//build/config/win:console" ]
     configs += [ "//build/config/win:windowed" ]
     deps += [ "//sandbox" ]
   }
 
   if (is_chromeos) {
     deps += [ "//device/bluetooth" ]
   }
 }