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..ebe6c9c00671ae0c12a437da6e7446a33c1961f2 100644
--- a/ui/resources/BUILD.gn
+++ b/ui/resources/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
+import("//build/config/features.gni")
 
 # GYP version: ui/resources/ui_resources.gyp:ui_resources
 group("resources") {
@@ -35,6 +36,11 @@ grit("ui_unscaled_resources_grd") {
   ]
 }
 
+webui_grit_defines = []
+if (enable_polymer_v08) {

[brettw, 2015/05/04 17:06:32]

Are we expecting this flag to be used all over the place or just here? What's
the lowest-level component that knows or should control this setting?

In the GN build we try to minimize the global flags. If you just want a flag in
one file for example, you can put a declare_args block in this file. The
features.gni file is really a hack for legacy GYP flags.

[mark a. foltz, 2015/05/04 17:55:19]

Maybe it could go into grit_rule.gni, and the define added automatically to all
grit() invocations. 

+  webui_grit_defines += [ "enable_polymer_v08" ]
+}
+
 grit("webui_resources_grd") {
   source = "../webui/resources/webui_resources.grd"
   outputs = [
@@ -43,6 +49,7 @@ grit("webui_resources_grd") {
     "grit/webui_resources_map.h",
     "webui_resources.pak",
   ]
+  defines = webui_grit_defines
 }
 
 if (!is_mac) {