Add the enable_polymer_v08 flag for gn. Equivalent gyp change was

ui/resources/BUILD.gn

Index: ui/resources/BUILD.gn
diff --git a/ui/resources/BUILD.gn b/ui/resources/BUILD.gn
index b73e0db9c4a97c60d44f005305a693bd7d931841..b81dd3a723527c757c07308bef6448fee7e4eef6 100644
--- a/ui/resources/BUILD.gn
+++ b/ui/resources/BUILD.gn
@@ -5,6 +5,11 @@
 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 = [
@@ -35,6 +40,11 @@ grit("ui_unscaled_resources_grd") {
   ]
 }
 
+webui_grit_defines = []
+if (enable_polymer_v08) {
+  webui_grit_defines += [ "enable_polymer_v08" ]
+}
+
 grit("webui_resources_grd") {
   source = "../webui/resources/webui_resources.grd"
   outputs = [
@@ -43,6 +53,7 @@ grit("webui_resources_grd") {
     "grit/webui_resources_map.h",
     "webui_resources.pak",
   ]
+  defines = webui_grit_defines
 }
 
 if (!is_mac) {