Reland "Add wrapper around GDBusConnection."

This is a reland of commit 50d8e75ae0bdaa4db8111a1a45df7ba64cd902d4

Fixes GN error when building on Linux with the remoting host disabled.

Original change's description:
> Add wrapper around GDBusConnection.
>
> Allows using standard Chromium sequenced callbacks with weak pointers instead of having to worry about GLib thread contexts and GCancellables. Also supports providing call specifications for type-checked calls.
>
> For unit testing, this reuses the D-Bus test service in //dbus, which is pulled out into its own source set so it can be included independently of dbus_test_server.
>
> Change-Id: Ib4b87ab3aec22b1fb62b32178461e4b9dbd6b70a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5809854
> Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
> Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
> Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
> Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1368809}

Change-Id: I452360f89d1e039687769e8d450dab4340c8c200
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5935954
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369666}