cr-os: add a script to fetch Chromium's build structure

This uses `gn` to gather a mapping of {gn_target: source_files}, with
some additional metadata attached.

This is intended to be used with the infra landed in cl/304465353.

BUG=chromium:908661
TEST=Ran to generate JSON files.

Change-Id: I091ee0cf9ef9a52eac25855ff83fabb7539aa8e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2136075
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
3 files changed