Add the enable_polymer_v08 flag for gn. Equivalent gyp change was

ui/resources/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("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
+declare_args() {
+  # Enable to include Polymer 0.8 in the binary.
+  enable_polymer_v08 = false
+}
+
 # GYP version: ui/resources/ui_resources.gyp:ui_resources
 group("resources") {
   deps = [
     ":ui_resources_grd",
     ":ui_unscaled_resources_grd",
     ":webui_resources_grd",
   ]
 }
 
 grit("ui_resources_grd") {
@@ skipping 10 matching lines @@
 
 grit("ui_unscaled_resources_grd") {
   source = "ui_unscaled_resources.grd"
   use_qualified_include = true
   outputs = [
     "grit/ui_unscaled_resources.h",
     "ui_unscaled_resources.rc",
   ]
 }
 
+webui_grit_defines = []
+if (enable_polymer_v08) {
+  webui_grit_defines += [ "enable_polymer_v08" ]
+}
+
 grit("webui_resources_grd") {
   source = "../webui/resources/webui_resources.grd"
   outputs = [
     "grit/webui_resources.h",
     "grit/webui_resources_map.cc",
     "grit/webui_resources_map.h",
     "webui_resources.pak",
   ]
+  defines = webui_grit_defines
 }
 
 if (!is_mac) {
   copy("copy_ui_resources_100_percent") {
     sources = [
       "$root_gen_dir/ui/resources/ui_resources_100_percent.pak",
     ]
     outputs = [
       "$root_out_dir/ui_resources_100_percent.pak",
     ]
@@ skipping 81 matching lines @@
     "$root_gen_dir/ui/strings/app_locale_settings_en-US.pak",
     "$root_gen_dir/ui/strings/ui_strings_en-US.pak",
   ]
 
   output = "$root_out_dir/ui/en.lproj/locale.pak"
 
   deps = [
     "//ui/strings",
   ]
 }