build: Create a chromeos GN args dir for device builds.

A gclient hook would populate the contents of this dir. You could then
re-use these files in any arbitrary normal GN build flow. (No Simple
Chrome shell needed.)

Bug: 937821
Change-Id: I30a78d491088351756017a77fe160de39296b7d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052205Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746499}

build/.gitignore

@@ -4,6 +4,7 @@
 ciopfs
 /android/bin
 /android/binary_size/apks/**/*.apk
+/args/chromeos/*.gni
 /config/gclient_args.gni
 /cros_cache/
 /Debug

build/args/chromeos/README.md

+This directory is used to store GN arg mapping for Chrome OS boards.
+
+The board listed in your .gclient file's `"cros_board"="some_board"` custom_vars
+variable will have a corresponding .gni file here, populated by a gclient hook.
+To use these files in a build, simply add the following line to your GN args:
+```
+import("//build/args/chromeos/${some_board}.gni")
+```
+
+That will produce a Chrome OS build of Chrome very similar to what is shipped
+for that device. You can also supply additional args or even overwrite ones
+supplied in the .gni file after the `import()` line. For example, the following
+args will produce a debug build of Chrome for board=eve using goma:
+```
+import("//build/args/chromeos/eve.gni")
+
+is_debug = true
+use_goma = true
+goma_dir = "/path/to/goma/"
+```
+
+TODO(bpastene): Add list support to gclient and allow multiple boards to be
+specified in the .gclient file.