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This CL converts ExternalService and ExternalConnector
to new Mojo types.

It also updates RegisterServiceInstance and Clone from
connector.mojom and members and methods which implement
them.

Bug: 955171
Change-Id: I8b9c6ab70a65b8c5661b294ec0ccde742123522a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803940
Commit-Queue: Julie Kim <jkim@igalia.com>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697446}