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This CL adds |public|, |mesage_port_fuchsia|, and |message_port_cast| to be
used for specifying a implementation for platform agnostic testing.

Bug: Internal b/168598365
Test: bindings_manager_cast_browwsertest, named_message_port_connector_fuchsia_browsertest
Change-Id: I4abfbb6a5bcfc19dbddb7634543a72f5bd288fdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489024
Commit-Queue: Jiawei Li <lijiawei@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822487}