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To prepare for adding new channel types (ie. fast posix channels) we
will need a way to extend existing Mojo NodeChannel messages.

Bug: b:173022729
Change-Id: I56b80ed58f28d058927d94ef9421319fe547c719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2538166
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#827995}