[Material][Mac] Replace hard-coded default favicon with .icon file

ui/gfx/vector_icons/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.
 
 action("aggregate_vector_icons") {
   visibility = [ ":*" ]
 
   script = "aggregate_vector_icons.py"
 
   icons = [
@@ skipping 15 matching lines @@
     "checkbox_normal.icon",
     "chrome_product.icon",
     "close_all.icon",
     "code.icon",
     "combobox_arrow_mac_disabled.icon",
     "combobox_arrow_mac_enabled.icon",
     "cookie.icon",
     "crashed_tab.icon",
     "credit_card.1x.icon",
     "credit_card.icon",
+    "default_favicon.icon",
     "eol.icon",
     "error_circle.icon",
     "extension.icon",
     "extension_crashed.icon",
     "file_download.icon",
     "file_download_incognito.1x.icon",
     "file_download_incognito.icon",
     "file_download_shelf.icon",
     "find_next.1x.icon",
     "find_next.icon",
@@ skipping 230 matching lines @@
     "../vector_icon_types.h",
     "../vector_icons_public.h",
   ]
 
   deps = [
     ":aggregate_vector_icons",
     "//base",
     "//skia",
   ]
 }