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Updates some stale system API references, adds new stuff for invitations
and platform APIs, and deletes a bunch of EDK crap.

Also adds a new convenience API to PlatformChannel and uses it in a few
places, because it makes example code in the documentation less ugly. :)

Bug: 844763
Change-Id: I91fbff53659dc87fc6fd33a2e5a774c419ba87a7
Reviewed-on: https://chromium-review.googlesource.com/1110954
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570310}