GN: add is_official_build

gn/BUILD.gn

Index: gn/BUILD.gn
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index fd2d0074c6f9abadeccf65b71206b99fa2147560..75f8352e936e1c9a11e90f775a1dc1190ea03d0a 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -28,6 +28,16 @@ config("no_rtti") {
   }
 }
 
+config("debug_symbols") {
+  # It's annoying to wait for full debug symbols to push over
+  # to Android devices.  -gline-tables-only is a lot slimmer.
+  if (is_android) {
+    cflags = [ "-gline-tables-only" ]
+  } else {
+    cflags = [ "-g" ]
+  }
+}
+
 config("default") {
   cflags = [
     "-O1",
@@ -56,14 +66,6 @@ config("default") {
   ]
   ldflags = []
 
-  # It's annoying to wait for full debug symbols to push over
-  # to Android devices.  -gline-tables-only is a lot slimmer.
-  if (is_android) {
-    cflags += [ "-gline-tables-only" ]
-  } else {
-    cflags += [ "-g" ]
-  }
-
   if (current_cpu == "arm") {
     cflags += [
       "-march=armv7-a",