Create gn arg templates for use with blimp builds. Update build doc.

build/args/blimp_engine.gn

Index: build/args/blimp_engine.gn
diff --git a/build/args/blimp_engine.gn b/build/args/blimp_engine.gn
new file mode 100644
index 0000000000000000000000000000000000000000..22a5e439fa8dceb5aba1206a294e1d0f5a716602
--- /dev/null
+++ b/build/args/blimp_engine.gn
@@ -0,0 +1,17 @@
+# GN args template for a blimp engine. Works within a docker container.
+#
+# Add import to arg.gn in out directory and run gn gen on the directory to use.
+# E.g. for out directory out/foo:
+# echo "import(\"build/args/blimp_engine.gn\")" > out/foo/args.gn
+# gn gen out/foo
+#
+# Use gn args to add your own build preference args.
+
+use_aura = true
+use_ozone = true
+
+# Not available within docker container.
+use_alsa = false
+use_pulseaudio = false
+use_cups = false
+use_glib = false