Split chromeos/network out of chromeos

Network is by far the largest subdirectory of chromeos.
This gives it its own BUILD.gn file.

Also added a ":chromeos_implementation" config.
Better to use a single config than a define each time, less chance for
undetected spelling errors.

Bug: 446937
Change-Id: I2f26cf3f9f15c4b4db364d7d2bc6cf5cc7e57887
Reviewed-on: https://chromium-review.googlesource.com/c/1276568
Commit-Queue: A Olsen <olsen@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599216}
7 files changed