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Refactors NamedMessagePortConnector into a platform-agnostic component,
which can be reused across FIDL, in-process, and testing contexts.
It uses Blink messages and message ports as a common basis for
message and port transport.

* Implements some missing portions of the Blink/FIDL MessagePort
  conversion layer.
* Adds a Fuchsia implementation of NMPC.

Bug: 1104369
Change-Id: I01e86c38963bfb114a7466c6f73b143e5302ad1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293110
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806684}