docs: Add a doc for D-Bus MojoConnectionService Provider

To increase the doc (go/mojo_connection_service) and the service
provider's visibility, this commit adds a doc in Chromium

BUG=b:175283521
TEST=viewed the md on Google Chrome locally

Change-Id: Ic9ceaecae0699bc2441a6cecf983f5fd156a48e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2583676
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Mike Pinkerton <pinkerton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858517}

docs/README.md

@@ -396,6 +396,8 @@ used when committed.
     patterns for practical IPC conversion work
 *   [Mojo “Style” Guide](security/mojo.md) - Recommendations for best practices
     from Mojo and IPC reviewers
+*   [D-Bus Mojo Connection Service](dbus_mojo_connection_service.md) - A service
+    in Chrome to bootstrap CrOS services' Mojo connection.
 
 ### Speed
 *   [Chrome Speed](speed/README.md) - Documentation for performance measurements and regressions in Chrome.

docs/dbus_mojo_connection_service.md

@@ -0,0 +1,50 @@
+# D-Bus Mojo Connection Service
+
+## Overview
+
+D-Bus Mojo Connection Service in Chrome is a D-Bus service that helps to
+bootstrap CrOS services' Mojo connection.
+
+## Bootstrap a new CrOS service
+
+D-Bus Mojo Connection Service lives in [//chrome/browser/chromeos/dbus/mojo_connection_service_provider.h](https://chromium.googlesource.com/chromium/src.git/+/master/chrome/browser/chromeos/dbus/mojo_connection_service_provider.h).
+
+Follow the example of CrOS Sensors' [changelist](https://chromium-review.googlesource.com/c/chromium/src/+/2352298).
+
+### Steps to Add a usage for a CrOS process with a new D-Bus method:
+1. Add a method name & a well-known token in both CrOS platform and Chrome.
+   (Recommend: `platform2/system_api`)
+2. Add the busconfig policy in [MojoConnectionService.conf].
+3. Upon a D-Bus request coming from the CrOS service, pass one endpoint of the
+   generated Mojo pipe to the component in Chrome that needs a Mojo channel to
+   the CrOS service. Ex: [RegisterServer](https://chromium-review.googlesource.com/c/chromium/src/+/2352298/16/chrome/browser/chromeos/dbus/mojo_connection_service_provider.cc#74) in CrOS Sensors' usage.
+4. Respond to the D-Bus request with the other endpoint of the generated Mojo
+   pipe. (Recommend: use the helper function [SendResponse](https://chromium-review.googlesource.com/c/chromium/src/+/2352298/16/chrome/browser/chromeos/dbus/mojo_connection_service_provider.h#75))
+
+The Mojo pipe can also be generated in the CrOS process, and pass the endpoint
+of it as the D-Bus argument to the service provider, instead of allowing Chrome
+to generate the pipe.
+
+### Steps to Add a usage for a CrOS process with an existing D-Bus method:
+1. Add the busconfig policy in [MojoConnectionService.conf].
+
+And that’s it. The method name, token and the logic in the service provider can
+be reused.
+
+## Security
+
+UID filtering should be used to ensure only the needed processes are calling
+the specific D-Bus methods, as processes/applications calling D-Bus APIs are
+trusted (written and reviewed by Chromium/CrOS teams) and should have a
+well-known UID to be filtered.
+
+UID filtering: Define access permission for each UID in
+[MojoConnectionService.conf]. Only the processes run under the specific UIDs can
+send respective D-Bus requests to the service provider.
+
+Arguments/tokens in D-Bus methods are still available if needed, which should
+be enough for multi-login situations and handling failures. The arguments can
+also be used to determine if Chromium should accept the request, and which Mojo
+interface should be used to establish the Mojo channel.
+
+[MojoConnectionService.conf]: https://chromium.googlesource.com/chromium/src.git/+/master/chrome/browser/chromeos/dbus/org.chromium.MojoConnectionService.conf