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This is a first step to convert the remaining old IPC messages
in content::PeerConnectionTracker to Mojo. At this time, this CL
converts PeerConnectionTrackerHost_AddPeerConnection.

The remaining messages will be converted on follow up steps.

This paves out the way to Onion souping peer_connection_tracker.cc|h.

BUG=787254
R=dcheng@chromium.org, guidou@chromium.org, haraken@chromium.org

Change-Id: I578bcd21ce68253377be853a5b05876bfa04aa13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823619Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#700177}