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This CL introduces AshChromeService, which is a set of APIs that
lacros-chrome will call into.
With this CL, Mojo connection between ash-chrome and lacros-chrome
is now bidirectional.

Bug: 1094106
Test: Ran locally.
Change-Id: I169d55ad964073e8faebe069917a6a3a67ed5c84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254638Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781662}