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This CL adds a declaration for ChannelPosix so it can be extended. This
will keep things cleaner when we add a Fast PosixChannel, otherwise
things would become unweildy in a single file.

Bug: b:173022729
Change-Id: Ie4ed21b885a62e4d755ae393484aa2717e0668b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586045
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#840261}