Helper for writing command line tools that have sub-commands.
Will be used by the CTS tool (roll, results, export, etc).
Bug: dawn:1342
Change-Id: Id22beecbc02b73af23064adc67c590f825b37789
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/88315
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Ben Clayton