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It currently defaults to the 'release' image, which is internal &
Google-only.

We have some public testers that need to flash to the 'full' images.
Adding the flag should let us do that by passing down the right type
of image based on GN args, which get bound to tests via generated
wrapper scripts.

Bug: 1057152
Change-Id: Ib1a82eb16ca274fe1e04946b588ec2dc2e049c77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303981
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#790091}