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

build/args/blimp_client.gn

Index: build/args/blimp_client.gn
diff --git a/build/args/blimp_client.gn b/build/args/blimp_client.gn
new file mode 100644
index 0000000000000000000000000000000000000000..f049dad2b18a720551a9c937c3b6b37f5b5239e1
--- /dev/null
+++ b/build/args/blimp_client.gn
@@ -0,0 +1,11 @@
+# GN args template for blimp Android client.
+#
+# Copy to arg.gn in out directory and run gn gen on the directory to use.

[brettw, 2016/03/03 18:35:56]

This should say run "gn args out/mydir" and type:
  import("//build/args/blimp_client.gn")
Users should not copy these files.

[Jess, 2016/03/03 18:49:04]

Updated this line to reflect the example below that has the same effect.

+# E.g. for out directory out/foo:
+# echo "import(\"build/args/blimp_client.gn\")"  > out/foo/args.gn
+# gn gen out/foo
+#
+# Use gn args to add your own build preference args.
+
+target_os = "android"
+is_component_build = true