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

build/args/blimp.gn

Index: build/args/blimp.gn
diff --git a/build/args/blimp.gn b/build/args/blimp.gn
new file mode 100644
index 0000000000000000000000000000000000000000..7404159dd54b2c140f2691fbd457d4079d866f07
--- /dev/null
+++ b/build/args/blimp.gn
@@ -0,0 +1,7 @@
+# Base GN arg template for blimp.
+# Shared development settings for blimp engine and client.
+
+is_debug = true
+is_clang = true
+symbol_level = 1  # Use -g1 instead of -g2
+use_goma = true

[Dirk Pranke, 2016/03/02 23:57:32]

I don't really think we should check this file in as-is, for the following
reasons:

1. None of these flags are specific to blimp.

2. is_debug=true and is_clang=true by default on Linux (i.e., you don't need to
specify these) (is_clang=false on Android, so that might actually be confusing).

3. I'm not really sure what you think you're saving by using -g1 instead of -g2

4. If I saw 'blimp.gn' I would assume that this was "the settings I want to use
to build blimp". But, apparently, that's ambiguous. If you really want to check
in something like this, I would call it blimp_dev.gn or something.