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These are cleaned-up versions of the EDK's PlatformChannelPair and
NamedPlatformChannelPair, which will be removed but remain untouched for
the moment. Most of the logic is duplicated verbatim, modulo cleanup.

For test coverage, since actual I/O operations are not being ported to
the public library, the new types are used in place of EDK types within
the invitation API tests.

Bug: 844764
Change-Id: Ieeea396370e37d4fa3a0e9a65b6f9b71855b85fc
Reviewed-on: https://chromium-review.googlesource.com/1068652
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561624}