Mojo: Consolidate Mach port brokering logic

Moves all Mach port brokering stuff into the Channel implementation,
making it a bit easier to understand. Previously relevant logic was
spread over Channel, NodeChannel, and NodeController.

This is in preparation for introducing a new PlatformHandleInTransit
type to encapsulate a PlatformHandle + ScopedProcessHandle, as a
replacement for the weird and sort of unsafe
ScopedInternalPlatformHandle usage we have today.

Bug: 753541
Change-Id: I7c17fb75d5930d8a8816a2927c8529d75c8b7675
Reviewed-on: https://chromium-review.googlesource.com/1112926
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570947}
14 files changed